Microsoft has largely decoupled their software distribution from their software licensing. You can actually download Windows 7 from their website, at this point. It just doesn't come with a license key. This seller may simply be offering to hand you an image of the install media. Note that nowhere does it say that it includes a license key. I'm sure this is still frowned upon, but it might be legal enough to for the seller to get away with.
Regardless, if the seller allows it to go for anything close to that price, it's pretty much guaranteed to either be incomplete (data only), a scam (you get nothing or nothing usable), or pirated.