If it's a vaporizer that you're after, check out the seller's posts and take note if the seller posted anything about problems with the unit prior to putting it up for sale.
Also refer to the manufacturer's site regarding warranty. I've bought used a few times through FC and never had a problem and I think I've always used paypal. Pretty sure I bought a used Vapor Brothers from FC
and it eventually needed a repair and luckily VB has an excellent repair service policy and the cost was minimal.
Somewhat unrelated but I've also bought used out of state from craigslist (there's a nifty firefox add on that searches craigslist globally) using paypal and generally didn't have problems but it's obviously more risky.
Best purchase via global craigslist using paypal wasn't a vaporizer (this part is definitely unrelated) but a pristine restored set of Dynaco A25 speakers with home made wood stands. Amazing speakers, professionally packed, one of the best online transactions in my experience, but I digress.