Don't buy a used one. Too many guys that own them neglect to clean them properly and promptly after using them. There is a very good chance that if you buy a used one you will find that the bore is rusted beyond reasonable use.

That's a pretty broad statement. I have seen plenty of high quality used blackpowder guns for sale, even bought a few myself. Just know what to look for, or ask someone who knows to have a look.

Kinda like buying used vechiles.

^^^^^^
Just speaking from my experience. Assuming the OP is unfamiliar with black powder guns and their unique characteristics; used black powder guns are much more likely to be found with rusted bores than modern firearms. Buyer beware.

Waggler is right in that a rusted bore is much more likely in a BP gun. On the other hand throat errosion is what afflicts cartridge guns but not MLs. Like K snow said, you have to know what to look for. It is a lot easier to take a look down a ML bore than to run a borescope into a cartridge gun to check the throat.

I know guys that clean their guns at rendezvous before a shoot because it was never cleaned after the last rendezvous shoot. (Smoothbore shooters mostly.) I also know guys, me included, that always clean the same day the gun was shot.

it is very much like buying used vehicles you have to know what you can fix and what it will cost and leave yourself budget for the unexpected you couldn't see
carry a flashlight and know what your looking for it wouldn't hurt to carry a few patches in your wallet.

some people do need to stick to buying cars with warrantees because they don't even know what they , don't know.