Buying used is a gamble, especially from individuals where no warranty is offered. The odds are in your favor since most cameras/lenses don't have problems, but you have to balance the lower cost against potential repair costs. I've bought quite a few used leneses over the years and I've been lucky in that none of them have proved to be faulty. If I can make a substantial saving, I'll take the rsik. If the saving is small I usually won't.

Buying from B&H, Adormama or other reputable vendors of used gear you normally get a 90-day warranty from the vendor, which is certainly worth having. You also usually get a 14-day return period during which you can send the lens back if you don't like it. It's normally more difficult to do that when buying from an individual.

As for upgrading (i.e. adding a new camera), it depends what you're upgrading from. If you have a 40D, I'd say getting a 5D makes more sense than getting a 50D. If you're upgrading from a 10D or a 300D, then it's not so clear which would be best. That would depend on what kind of shooting you do. If you shoot mostly portraits and landscapes, then the 5D may give you better image quality in large prints. If you shoot sports and action then the faster frame rate of the 50D may be something you need.