It won't just be "sent forward". It will be opened and repacked.
Hopefully if you enclosed any identifying document they may be able to associate it with the correct transaction.
Otherwise, if you're lucky they may just return it to the PO marked "Not known at this address". Providing you wrote your name and address as sender on the parcel you should get it back.
I don't think there's any way to find out about specific parcels, but you could try asking eBay customer service. Otherwise, you can only wait and see.