I agree with the reasons given here for giving a guarantee. I disagree, though, that most people won’t try to refund. I hear a lot about serial refunders – people who buy the product, and either refund shortly after purchase or right before the 30 day period is up.
On the flip side, I recently had an issue with two product sellers. One whose product had a virus (caught by my anti-virus software) told me he could not refund because his PayPal account was on hold. Gee – I wonder why. So I had to file a dispute. The other one was this week – after detailing all the issues I had had with getting the product to work, I asked him to let me know if he was going to honor his guarantee. I got notice of the refund shortly after.
I think it is good to have a guarantee, and it is important that if you have one, that you don’t make anyone jump through more hoops than you told them they would need to, to get their money back. This is the problem I had with the latter person.