You say you have a receipt of posting, what kind? Was it sent, recorded or registered delivery? Either of these and the problem lies with the post office to find out where the item is.
If, however, you just have a receipt for purchasing stamps at the counter, you are open to be stung by unscrupulous purchasers who will prey on items correctly delivered to them , but with no obvious proof of delivery will "claim" they never received the item, hoping to get another for free.
I hate these type of scum.
Always always when selling on any auctiion site allow for recorded delivery costs, or if valuable, registered delivery.