I used to get an e-mail from eBay when a seller sent an offer on an item I was watching. That hasn't been the case lately.
The only way I know there's a pending offer is when I visit the site and see a notification there. I log in daily since I'm also a seller, but what about casual users who check eBay only periodically? I'm sure they are missing offers that go unnoticed and just end up expiring.
I should add that I don't have any restrictions on communication from eBay. So what gives with the lack of notification by e-mail? I used to routinely get the "Because you showed interest ..." messages. Needless to say, I don't always buy, but you can't accept an offer that you never saw.