The nearest I've had happening to this was during an eBay spending spree last year. I had a set of 5 model aeroplanes mysteriously arrive in my post. The address label had the eBay logo so it was obviously from one of my sellers. However, it was in addition to what I've already bought and had no connection items-wise with my other items. There was no "return-to" address label, no invoice, no eBay ID. Plus, because I bought from numerous sellers in a short space of time, there was no way of knowing who sent it, and thus I couldn't contact them about their error. To play safe, I waited a month, had no contact, so I just flogged the aeroplanes back on the 'Bay. Got about £25 for it.