A lot of gunmakers don't support a used gun.
A few, like FNH, will continue to honor a warranty for their weapons, regardless of whether the current owner is the the first or fiftieth owner.
The rest tell you that
1) they'll cover it if you're the original owner,
2) they'll cover it for X years if you're the original owner, or
3) sorry, you're not covered if you aren't the original owner.
In some cases, if the gun's not in current production, you might not even be able to get parts. (Saw some discussions recently where you can't get recoil spring assemblies for Ruger P-95s, and I've had some issues with getting sights for a 3rd Gen S&W 4506. If Numrich or Jack First doesn't have it, you may be S.O.L.)
I'd be unhappy if the OP's gun were mine, too, but I'd chalk it up to experience. (I had a similar experience with a Witness Sport Long Slide some years ago; I ended up buying a new barrel and a new SuperSight, neither of which were cheap or covered by warranty, or available in the aftermarket. I did nothing to cause the problems with those assemblies.)
Most of this -- how a gunmaker handles work on used weapons -- can be found/known before prospective owner makes a purchase or trade. If he or she doesn't investigate, that's his or her problem.