I would say it depends on the warranty. Are you sure the JL unit will work? Where are you getting it from. I think at the end of the day however the JL amp is probably still better.
I'm looking on ebay for the used JL. Of course the new JBL will have a full warrenty. I'm more than a little reluctant to buy a used amp. If the JL is so good why are people moving them after only a couple of months use? Do they take a lot of power consumpsion that requires a step up in your alternator which would be an additional cost?
I have no idea why they are sold but you should just buy from a reputable seller. You could say the same why do people sell their old school amps or high end Xtants, Zapcos, Audisons, etc.