Here's a feature that eBay needs to implement: a method for notifying a user that their e-mail address doesn't work. It sounds like a stupid idea, right? Well, it's not. Here's what happens, a user buys something at an auction, but their e-mail address with eBay is incorrect, and the user uses the eBay interface to mail the seller asking about the auction. The seller replies, to an address that bounces, and has no way to contact the buyer. The buyer e-mails again, via the eBay form, complaining about the seller's lack of response, and the seller is left powerless. What eBay needs is a way for a seller to flag a buyer account as having a bouncing e-mail address, so that the buyer is presented with a notification about it the next time they login, hopefully in an extremely non-subtle manner.
So, as it stands, right now there's a guy in Italy who'd really like to pay for my Nikon Digital Camera, but I don't have any way to get the shipping price to him because his firstname.lastname@example.org address keeps bouncing. Great.