Nearly every amp I currently own I bought on consignment. Generally, they were sold as-is, no warranty, no shop or seller responsibility for any issues once I buy it and it leaves the shop. Now, what my shop <DID> do for me was to let me try the amp, open it up, look for problems, and otherwise poke around if I thought there was a problem. Sometimes they'd let me take it overnight, which I thought was sweet! Usually there WAS a problem, and several times it involved safety. They would then call the owner, and have them either fix the amp, sell it at a much-reduced price, or please come pick it up.
I got a Bassman 100 for $150 because it came in "needs power tubes" (from a tech) which I knew could mean anything from "sounds funny" to "catches on fire." I asked if anyone had tried the amp, and nobody had. Turned it on, stand by off, fuse blows immediately. I guess it seriously needs power tubes... they called the guy, said they can't sell a broken amp... he said he wanted to get $100 out of it, so I offered $150. $70 and a few resistors later, I sold it for $400.
The only ones I never worried about were when the owner sold a personal instrument or amp. I ended up with a few of those over the years.