On your website, explicitly declare a N days refund guarantee. It's a selling point, shows good service and people generally don't abuse it. Worst case, you'll remove it.
It's actually logical - if after the demo you decide that you don't want the product you just don't buy.
If after buying the product you want a refund you still need to contact the seller (or go through some automated process). It's an extra step.
As a seller though, I would think less people would ask for a refund than the group of people that would have bought the product, but instead downloaded the demo and then decided not buy.
See number 7 in http://www.joelonsoftware.com/articles/customerservice.html