This tip is to help you become a more informed buyer of photographic art. There are a lot of things that go into the quality of a print besides your personal attraction to the image, and there are even unscrupulous sellers who exaggerate claims or conceal drawbacks. Of course you must adore an image before you even think of buying it, but what you learn here may help guide you through the thicket of variables to decide if the price being asked is fair.
How Was the Print Made?
The foremost consideration is how the print was made. There is a dizzying variety of means of producing a photograph, and some are much better than others. All photographs fade and discolor slowly over time, sometimes over centuries and sometimes over months, depending on how it was made. We won't know for sure how long a print lasts until it actually starts to fade, but accelerated aging testing (blasting it with bright light to make it fade faster) can make crude estimates of the display lifetime of a print. Take these test results with a grain of salt, but they do give some indication of the longevity of a print made by a given process.
I'll start with color processes. In the middle of the pack these days, in terms of display lifetime, are prints made in the traditional fashion, with enlargers projecting a film image onto photosensitive paper, which is then processed in a series of chemical baths. Even within this one category, there is a large variation of tested lifetimes. Every paper is different! The best seems to be Fuji's Crystal Archive paper, with an estimated time before noticeable color shifts in the 60 year range. Kodak historically has had much poorer papers. They have introduced a paper claiming to have a 100 year lifespan, but this claim is based on fading criteria four times looser than the Crystal Archive tests -- the Kodak lifetime would be 25 years under the same conditions that the Fuji paper yields a 60 year lifetime. The Ilfochrome (formerly Cibachrome) process is probably comparable to the Crystal Archive paper.
The digital variant of this classic process is to have an image exposed directly onto a photosensitive paper using three colored lasers, LED's or a CRT screen as the light source. The lifespan of these prints is exactly the same as the lifespan of the paper they're printed on, but the quality of the image is likely to be better due to the exquisite control allowed by digital processing. (Lightjet and Chromira are popular machines of this type.)
At the high end of the lifespan range are specialty prints made by expensive processes such as the carbon printing processes. In these prints, three color separations are made of the image and long-lasting pigment sheets are laid down in registration on a piece of paper. These can last centuries, but are expensive. You're not likely to run into one of these in your church bazaar!
Digital output using inkjet printers is all the rage these days, but beware: many of these prints are very short-lived. I had one posted in an east facing window to catch a few hours of direct sun in the morning. After six months, it was significantly faded. The problem is that many consumer inkjet inks are dye based, and these fade rapidly in bright light. A pigment based ink, on the other hand, lasts much longer. Epson's version of these inks, called Ultrachrome, when printed on certain papers, is tested to last even longer than Crystal Archive paper before noticeable fading or color shifts.
If the print is made on a color photocopier, I wouldn't even touch it.
Note that I have been talking of display lifetimes, i.e. lifespan when exposed to light. If your photo is up on the wall, this is the only lifespan to consider. But prints fade and discolor even in the dark. The processes that cause the dark color shifts are different from those that cause light fading, so the light and dark fading times are not directly related to each other. Beware of photographers that praise the lifetime of their work using only the dark storage numbers! If your photo is going to be on display, it is primarily the light fading that matters. To trump up the other figure in sales hype is disingenuous.
Black and white prints tend to last much longer than color prints. A properly processed traditional black and white print can last for centuries. Note that this assumes is a properly processed print. (For those who know what these terms mean, that includes printing of fiber based paper, using two fixing baths, using hypo-clear, selenium toning, and extremely long wash times.) In general, fiber based paper (FB) prints will outlive resin coated (RC) prints. Inkjet output of black and white prints lasts longer than color inkjet prints, even when using the same ink/paper combination. Thus the Ultrachrome inks, used with certain papers, is a good choice for printing black and white prints. There are carbon-pigment inks on the market that outlast even Ultrachrome.
To sum up: question the longevity of inkjet or photocopier prints and ignore claims of dark storage lifetime. Fuji Crystal Archive is the best traditional color paper available today. Lightjet or equivalent output which burns a digital image directly onto traditional paper won't last any longer, but is worth more because of the better image quality. Specialty processes are expensive, but give exquisite images with long lifetimes. Traditional black and white prints should receive proper processing.
How Was the Print Matted?
The matting is the thick board with a big rectangular hole in the middle that holds the photograph. It makes handling of the photo safer, since you're only touching the mat. It sets off the photo in the middle of a blank field, separating it from busy surroundings for a more pleasing display. A beautiful matting job can enhance the beauty of the photograph. And when placed in a frame, the matting spaces the photo away from the glass so it won't stick to it.
Mats come in a variety of qualities. The cheapest is poster board, simply glorified cardboard used for short term displays. This stuff is cheap, but it is laced with the natural acid of the wood pulp it is made from. This acid will eat away at the photo. If the photo is matted this way it won't last nearly as long as it would in an acid-free mat.
Good mat boards come in two kinds. One is a wood-based board, but with the acid (and wood lignin, also an evil agent) removed. Even better is a board made of cotton ("rag board"). Some mat boards are declared acid free when the lignin hasn't been fully removed, and it will turn acidic in time. So look for "acid-free" in the mat board, but the two boards I've just described go beyond simply acid-free.
If the print is framed, it must be framed with a mat. If the photo is just popped into a frame, it will eventually stick to the glass and be ruined in time.
To sum up: poster board makes a poor quality mat, acid-free and lignin-free wood pulp board or rag board are the best. If the photo is in a frame, it should be matted.
Let the buyer beware: there are a couple of other things to watch out for when buying. One is the size of the print. If you comparison shop, be advised that some photographers list the "size" of their print by the size of the mat board it is mounted on, not by the actual photo size. This gives them a false appearance of economy. Be sure you know which your particular vendor is using. There is nothing wrong with carrying around a ruler to check the size of the photos for yourself when you visit a gallery or artist's display.
Lastly, let me point out to you that prints can look different under different illuminations. A print that looks muddy and dark under dim light may spring to sparkling life under a brighter light. Prints viewed under the yellowish light of an incandescent bulb may look different when viewed under sunlight coming through a window. If at all possible you should view a potential purchase under the same illumination, both color and intensity, that it will have when you display it. When buying in person, don't be afraid to ask the photographer if you can hold the photo and walk around into a light source like the one you'll be hanging it in. Since this shows you are an informed buyer who is seriously considering a purchase, they should have no objection to this.