I'm pretty sure it means "if you buy it, don't sell it as your own and keep teh money" I know with PLR stuff it means "one owner only" - kinda like on istockph0to or some template websites where you can buy the rights to a photo/template, and they remove it from their site so no one else can buy it.
I think it means no double pushing offers, like most of the flogs do... I got an email from C2M about double offer pushing, but it didn't use the "Double Sell" term, so who knows. Ask you AM, they will trust you more and feel you want to comply with their terms.
I would assume that it is referring to trying to sell a product to someone...and then attempting to sell a similar product to the same person. That is just what I am gleaming from the two words "double sell" though. I'd read the fine print and talk to your AM for sure.