I've been working with my gf for the past few years selling plants at nearby farmers markets, ranging from small 10 vendor markets to a massive outdoor market that sees upwards of 10,000 customers at the peak of summer. Last year I started selling animal and plant themed photo cards as a little side project. They are printed as 4x6's, matted on cardstock, stuck on a 5x7 card and wrapped in a crisp plastic bag. That's my background in this, here's my advice on what I've learned:
-I would check around local art shops and galleries to see what kind of prices people are asking for unframed matted prints when you're trying to determine a reasonable price. There's really no right answer for this, it depends on the area you're in, the other competition, and of course how popular the stuff you're selling is.
-Once you set your price, never haggle on your artwork. People at markets often like to haggle, but in my opinion it greatly cheapens things like art. So does putting your artwork on any kind of large sale.
-Don't do this expecting to make much money
. My overhead is essentially zero as we'd be at the markets anyway, and I never count my time taking or processing images as a cost as I'd be doing that anyway for fun
. These aren't custom made photos for someone, and once it's print ready, it's print ready for life.
-Absolutely get clear bags for your work. Mandatory at an outdoor market, but it looks much slicker in any case.
-Farmers markets can be a relatively lousy places to sell artwork. The enormous market we go to has in part become a huge flea market and everyone is looking for cheap deals. I did just as well at it as some of the small 10-30 vendor markets I go to that had a much smaller but different category of clientele. I also tried a small 'Art in the Garden' show which had maybe 80 people show up who were specifically there with an interest in artwork and card sales were way better, enough that it was worth it being there even though I was only selling cards.
-Consider also selling cards and smaller prints. These are more likely to be impulse buys at a market than larger stuff.
-I've never gone to a farmers market that asks for a % of daily sales, and honestly I don't think I would as this just seems wacky to me. They aren't art galleries where someone else sells your stuff and gets a commission, but more like temporary shopping malls where vendors are responsible for selling their own stuff. But farmers markets may operate differently in your area.
-As Miguel says, definitely view praise from family and friends as suspect. But also keep in mind that what other photographers like isn't necessarily directly correlated with what joe public will buy. People who identify themselves as photographers don't generally buy other photographers prints in my experience.