Just look it over real good and make sure there is no rust. Also, have the seller hook it up and run it through a cycle. Make sure it doesn't make any abnormal sounds (normal agitation goes like thumpa-thumpa-thumpa-thumpa), and make sure it neutral drains and successfully spins without any grinding or ratcheting sounds. Oh, and if it has a dual action agitator (spiral shaped), check and make sure the top half can only rotate in one direction independent of the bottom half.
If the machine doesn't mechanically check out for any one of those things, don't worry. Direct drives are incredibly easy to fix and parts are still dirt cheap.
As for the exact age of these machines, the only way to tell is by getting the model and serial numbers. If I had to guess, I'd say late 80s to early 90s.