Quite a few members have asked the same question recently. I wonder if there's some glitch in eBay's addresses database.
What you haven't told us is whether it was addressed to you, or to someone else?
If to you, and there's no return address, I'd just keep it for a while in case the sender discovers their mistake. If you don't hear anything, as the trousers are your size I think you can treat yourself to a Christmas present!
If it was addressed to someone else, it isn't your problem. Normally I'd say return it to the PO as not known, but as you've already opened it and there's no return address this would be a waste of time. It would just end up in a lost post auction. I don't think you need feel too guilty if you do as above, but perhaps allow a bit more time for someone to claim it.
