There is not a lot you can do through Paypal if you chose not to register the large letter, it would have been three bucks well spent.

If you are sure it is the same item you sent that is being sold on Gumtree you could try telling them that if they don't pay up pdq you will be going to the police but unless you have a seriel number or someother exclusive identifier it would be impossible to prove.

If your receipt has the buyer's postcode on it then you may be able to convince Paypal but I very much doubt it.