I am using a Macbook Pro I bought from eBay. It was and is like new.
Pay attention to the seller's ratings. Make sure you know what you're buying and it's market value. I won't take the risk on a seller with only a few sales and marginal feedback. I try not to use sellers under 98% positive feedback. I look at the feedback also.
If you're diligent, buying on eBay is safe. Insist on actual photos of the machine, ask questions if you have concerns, and enjoy if you buy!