I spend aroung 30 weekends per year selling my artwork at outdoor art shows. It's not exactly the same clientelle as swapmeets, but some of the same principles apply. Here are a few things I've learned:

People don't like to be pressured, but they do like to be acknowledged. Try to greet everyone. A simple "hello" and a smile is usually sufficient. Your customer is there to shop, not talk to you. If he or she finds you to be a particularly compelling person, he or she will instigate further conversation.

Talk to young children. Nothing endears you to Mom or Dad quite like having them believe you think their kids ar cute.

Get some stickers of some sort and price everything where it is plainly visible. Many people simply won't ask how much an unpriced item is and will walk away.

If you're behind a table, either stand up or sit on a high stool of some sort so you're at the same level as your customer. On a subconscious level, rising up from a regular chair is a threatening gesture and it makes people feel uncomfortable. That's why most artists at art shows have bar-stool height director's chairs.