I would imagine that the purchase on ebay would be made in terms of USD. At the moment of purchase, the BTC equivalent would be locked in and immediate payment would be required.
ebay's policy currently has some leeway in allowing buyers some time in paying for their winning bid as they would not necessarily be at their computer when the auction is over. If ebay were to allow bitcoin payments for auctions then I agree that the auction would likely be priced in dollars, but the btc/usd price would likely be effective when payment is received. It would also be likely that payment would be made to an ebay escrow address and the buyer will need to confirm receipt of the goods prior to the coins being released to the seller