Flash memory has a certain number of write cycles it can handle before it can't be written to anymore. My concern buying a used SSD would be that someone put a LOT of use on the drive and was just trying to sell it before it died.
How much of a discount are you getting on the used drive?
I'd stay away from the first generation products. Would be very selective with which brand/controller I'd buy and probably wouldn't look at anything older than 12-18 months anyway.
A few months ago I bought a new 256GB Crucial M4 on sale for $165 and a new 256GB Samsung 830 for $162. You can save a decent amount of money just shopping around for good deals.