I think sometimes the party hostess gets a bonus or something based on how many people attend, even if not everyone buys. You can look at the stuff and take a catalog to look at. Usually they keep the party "open" online after the event for people who want to buy later, so not everyone has to feel obligated to buy something right then and there. Then just..don't order anything later either. I don't think its rude. What if someone went with plenty of money and didn't like anything enough to buy it?
A lot of my friends have all kinds of parties. Sometimes I buy something, sometimes I don't. I usually have fun though.