Winning may be a correct term in this context, but there is no doubt that e-bay hypes its language in e-mails to encourage bidding and buying.
"Don't let it get away" and the like--there's an adrenalin rush from winning, losing, or risking things getting away that just aint there with simply paying, purchasing or being the high bidder.
And 'winning' in the context of 'buy it now' sales is simply a misnomer!

That my two cents--but don't let this one get away! Bid again!--you'll win if you put in a bit more.