Ordered an item on eBay that turned out to be a bit small, spoke to seller to see if they’d be happy to do a straight swop for a bigger size. Seller asked me to return item, and they’d just issue a refund. Ok fair enough. Posted item off 1st class two weeks ago and seller is saying it still hasn’t arrived. I have proof of postage that I have sent item off. Do I raise a claim with the Post Office as my first port of call - I’ve now got no item or a refund. 