I doubt they will support it but its not as bad now they are automatically adding keys to bnet accounts. Its mostly keys handed out to media groups /websites that can end up on ebay. People with SC2 and other games on their bnet account might not want to sell those along with the D3 beta.
As far as ebay goes, I'm sure they are taking down beta keys these days right? GGG (the PoE dev's) posted 100 beta keys on their with all the proceeds to be given to charity..a nice idea I thought but ebay removed the listings.
Maybe some slipped through the net with SC2, maybe ebay will miss them for D3 also..but even so people will always find a way, even if its a simple direct paypal transfer which Blizzard could never hope to police.
Its something they will try to prevent, but I can't imagine they devote huge resources (and cash) to ensure that.