I'm curious...did
ebay actually tell you that your offer had expired or the "item got away" type of thing, or did you just assume so because the listing had ended?
It physically shouldn't be possible for the seller to accept an offer that expired because that option would no longer be there for him to click on to accept.
It sounds like you assumed the offer ended because the listing ended, but unless it was an auction, I believe the listings don't actually "end" but just get relisted, as if it's simply refreshed for another 30 days or whatever.
Either way, I would contact the seller and let him know you thought the offer expired and the listing ended. I'm sure he'll understand.