Dino
I personally wouldn't steal anything. If something interested me, I'd buy it, and deposit my cash in the cash box. I wouldn't fill out the guest book though.
From what I understand of your scenario, the situation is different from a "pay what you wish" deal, where there are no set prices for things. In that case, you're counting on the person's altruism, and various other factors. There's an interesting article here in case you're curious:
http://www.getelastic.com/name-your-own-price/
Note that in the article, it says that when you give a suggested price, the vast majority of your customers will hover around that price. With your thought experiment, I'd assume that since the price is not suggested, you're talking about a much higher societal pressure to conform to those norms.
You're correct and I have the prices set for a reason. The article brings up a big different, I think, that needs to be taken into consideration for this scenario. The people are in fact interacting with the booth in person and not online. So, perhaps it's difficult to imagine NOT having people steal from you while online, but it's easy to think in a real life set up people would be mostly honest.
Would the fact that the artist/seller wasn't there to be the salesman affect your inclination for the product?