I have some pieces I've been displaying for several years. They attract attention but for some reason no one actually buys them. All the jewelry I sold Saturday was new, meaning made since the last show I did, but not all of the new stuff sold. At a show I did last spring, I sold nothing new, but sold three pieces that were at least four years old. You just never know what people will buy.
And as a vendor you have to learn not to take it personally. Sometimes that's very hard to do. But people aren't going to buy everything and it's not because they don't like it or they don't like you as an artist. Eventually the right person will come along at the right time with the money in hand, and the piece will go. Then you have to learn not to be so attached to the piece you can't say good-bye!
The main thing, though, is to keep perspective. Very few people get rich doing this, and the ones who do only get there through a lot of damn hard work. And it's easy to go broke doing this, too. In the past couple years I've watched several fellow artists go through bankruptcy and foreclosure and total loss of everything because they somehow or other got an inflated sense of their artistic worth and lost all sense of practicality.
I do this because I love rocks, I love sharing what little knowledge about them I have, and because it's fun.