When you're buying old gear, pick it up. Is it heavy? That's usually a mark of quality. If it is made of real things, like wood, metal and glass that's also a good sign. If you're buying speakers, take the grills off and check the cones. Make sure they're not brittle, and that there are no holes. I've set up several stereo systems this way. I always wind up giving them away to friends. Half the fun is in the building of the system. Once I've gotten it to the point where I say, "Wow, that's great sound! I can't believe I only spent $20 on that!" I remember I already have two stereo systems, and don't really need one for every room in the house.
I just started on a new one. The first part is a TEAC A400 tape deck from 1976, bought for $7. It has VU meters with needles that bounce instead of LEDs, and metal levers and knobs that turn instead of plastic push-buttons. It is a wonder to behold and a joy to fiddle with. One day I'll crack it open and clean out the rust and replace the belts and make it actually work again. Then I'll probably wind of giving it away, but that's OK because for $7, I'll have gotten my entertainment value out of it.