I've bought or swapped for a lot of motors over the years, and my approach is usually to assume its junk and pay accordingly.
The one time I bought a motor planning to just drop it in and go I eventually discovered someone had tried to remove a main cap with a chisel. The seller returned my money, but could not return the work I'd done installing it.
I have lots of old motors most of which I could not start without lots of work, if at all, and generally if I sell one its for rebuild, not as a runner.
A better question might be how do you decide if that motor sitting in the corner of the shop can be installed and run as is, or if it first needs to be rebuilt?
