Don't be afraid to offer less than the price they're asking. I buy quite a few things from pawnshops. They'll almost always come down on their prices. Naturally, the more expensive the item, the more room they'll have to come down on their price, though. If they're asking $50 for that lens, you might try offering them $30 and hope to settle for $35 or $40. Make sure the glass is good. No scratches on the surfaces...no fungus or dust on the internal elements. Look for oil on the aperture blades. You shouldn't see any or certainly barely any. Also check to make sure the aperture will stop down properly and that the aperture ring, focus ring, and zoom ring (if it has one) turn as they should. If all those things check out, the lens is probably okay. One other thing to look for is whether or not there's a filter on the lens. I've seen lenses that looked like they were really scratched up, but when you took the filter off, the glass underneath was in great shape. Uh...if you find one like that, be sure to put the screwed up filter back on and offer even less than you would otherwise because "the lens is all scratched!"
Good luck!