I wouldn't touch it. There's obviously something fishy with this, since any (functional) Early 2011 Macbook Pro would easily sell for 3 times as much literally everywhere else online (I would know, for the last 2 weeks I've been trying to find a cheapo Macbook for my sister to buy and the cheapest I can find, with lower specs, is around $500).
It's either stolen, has some sort of hardware defect, or has some other issue that makes it worth jack. I would say the seller doesn't know what it's worth, but that's obviously not true since it's supposedly being sold for "$799.95" at his "store".
Ask him for pictures of it functioning, ask for screenshots of each hardware info page (including battery life) and ask why it's so cheap. If he dodges any of those questions, something is fucked with it. FAST had the right idea above, it's also likely they just "fixed" some other defect and are hoping it won't die by the time the warranty ends.