craftgrrl | Tell me more about selling crafts!

It's been a while since I posted, but I have yet another question about selling crafts at craft fairs and the like. I'm thinking about signing up for a local fair in about a month where, says the coordinator, about 500 people come every year. Out of those 500, how many can I realistically expect to buy anything? I don't want to make too much stuff, but I don't want to run out halfway through the day either (and I can always sell a few extras on Ebay or online -- I just dont want to end up with wayyyy too much stuff lying around my house when I'm done). Everything I'm planning to sell (bandanas, headbands, and hair 'chopsticks') is going to be priced under $10, so I'm counting on the occasional impulse buy, but I still think I'd be lucky if I got 10% of the people who stopped by to actually buy anything. Is 10% a fair estimate or am I way off? Experienced craft-salers, enlighten me! :D

Also, today was a gorgeous day so I grabbed the scarf I'm working on and just sat out on my deck, knitting away. Man, I love spring. :)