Offer them a low low low price. They aren't worth much as an unknown quantity and where else is he going to sell them? Remind the seller that (a) they are a single, potentially catastrophic point of failure, (b) that you can't verify their condition or previous use, so (c) you're taking all the risk. Once you get them you can check them out properly - if they turn out rubbish, well you didn't lose too much did you.
If you're that worried, buy a new one.