I use to offer a very limited warranty.
If the buyer wished, I would test fire the gun before shipping it... If the gun worked before I shipped it, I would give the buyer a few days from when they received it to test everything out...
Only complaint I ever got was "The gun doesn't shoot" but that's because some one was trying to cycle a 400 FPS G&P with a 8.4 600mah NICD that was charged with a "blue box of death"
But... other types warranties... Not a chance in hell. There's far too much that can go wrong on the user-error side... and it's difficult to determine if the it was user error or not. (using a 10.8V battery and walmart BB's in a high end gun.. ect ect ect)