Once!!

...but obviously, it depends on what is at fault. Sometimes, there may be a difference of opinion over whether a fault even exists at all. I suspect that if the seller is confident that the fault is fixable, he should be happy to take it back for a full refund, knowing that there are other buyers lined up for any good double, and that its value may even have increased in the interim (in the case of a Brit gun).

The trouble starts when the seller believes the fault occurred after sale, perhaps due to the actions of the buyer.

Get a good lawyer...

FWIW, I would always prefer to fix any problem myself, or through my own gunsmith, then I know its been done right. Any serious fault should be detected during the buyer's inspection and test-firing sessions anyway, and factored into the agreed price.

If you've just paid ten grand or more for a DR, tossing a hundred bucks at a trigger-adjustment or stronger safety-spring is peanuts.

I'd be interested to know what kind of faults other posters have found to need multiple attempts to fix, that weren't able to be detected during the pre-sale inspection?