I believe that sellers have the opportunity to accept whatever payment methods they choose. If they can find a buyer who is willing to do this, they could sell their product for cash in the mail if they wanted to. I really don't see anything that would prevent people from paying with bitcoin now when they win an auction.
Mostly, the fact that putting "I accept bitcoin" in their auction would cause eBay to cancel it. This leaves no good way for a buyer who is willing to pay with bitcoin to find a seller who is wiling to accept bitcoin. Sending messages to each seller as a buyer would take a monumental amount of time when most of them probably aren't going to accept bitcoin, and hoping to get a message from a buyer offering to pay with bitcoin when most buyers aren't even aware of bitcoin wouldn't leave one with very good odds. Beyond that, I'm not sure how far eBay goes to enforce their terms, but since it's against the rules to negotiate and sell items listed on eBay to people who found them on eBay outside of eBay, I wouldn't be 100% surprised if a message sent on eBay about bitcoin could lead to an account suspension. However, people who want to buy and sell with bitcoin have other sites they can use, just without as much "protection," which the majority of users must think is free in spite of the fact that the sellers pay the fees and the buyers pay the higher prices. The level of protection offered now (mostly to buyers) didn't always exist. I've been scammed on eBay when paying through PayPal for physical goods that were never delivered. The seller withdrew funds and eBay and PayPal didn't actually offer refunds via insurance, only reversals when possible. Now, in order to get the protection everyone knows and loves, not only do the funds have to go through PayPal to ensure that PayPal and eBay get their cuts for the costs and/or risks they take providing protection, the fees have doubled and the funds also have to be escrowed for a long time for new sellers amongst other things. The same fees and escrow periods will be true of auctions that accept bitcoin. Allowing bitcoin to be sent directly to the seller would require the buyer to take all the risk, and most of the buyers on eBay don't want to do that. More importantly, though, the majority of posters in this thread and the "Amazon needs to accept bitcoin" threads believe that these big powerhouses accepting and handling bitcoin will make other people who are not users more comfortable with the idea of bitcoin, spurring adoption, so even though you could theoretically win an auction from a seller who agrees to accept bitcoin after the fact (or even before you bid), that doesn't provide the benefit that everyone else here is looking for.