If the payment is made from an instant funding source (such as PayPal balance, credit card or instant bank transfer) then the buyer does not have an ability to cancel the payment. (They would need to request a refund if they need to cancel an order.)
Assuming you are talking about flash demos as the product, its very difficult for a buyer to get a refund on an item like that. PayPal does not give refunds on downloadable products etc., but they do monitor complaints accumulated over time.
Assuming you are talking about flash demos as the product.....
No I am not talking about flash demos. I never have such issue. Actually I am thinking to sell China Products on eBay. However it seems that this kind of selling on eBay may be quite risky for the sellers.