Will laptop overheat because of this cover? http://www.lapstyle.com/image/obj157geo150pg6p15.jpg

Why not? If YES - how this product can be saleable?

Depends entirely upon your laptop and where your fans are, but even IBMs, where they have the fans on the sides, will overheat due to the heat-storing capacity the "fur" has.

Granted, it could be made of synthetic material that could withstand the heat... but I still think it would heat up slightly more than it normally would.

As far as your answer to things being marketable...

... two words. Pet. Rock.