He's trying to rip you off. OEM version of that card does not exist, and it is identical to cards from other brands. How much is he selling it for?
If anything, it will perform worse due to worse cooling an inability to overclock well.
Ahh well, i dont know what should i say that OEM or a ati own manufactured card. This card exist mate! i see my self. He's is selling at 90$. The same series of xfx card is also available at 80$.
Here is the review of Original Ati Manufactured HD 5770 Card.http://www.trustedreviews.com/AMD-ATI-Radeon-HD-5770_PC-Component_review
I don't know about the cooling of this card, well before buying i'll Definitely check the temps!