>>No because the seller doesn't get the postage charge. If I sell something for £10 + postage, I get £10<<

Do you mean you never see the postage charge ? If so why did the EBay operative I spoke to say "just put your own tracking number in and you won't get charged the carriage [by EBAY]". Are you saying that might be true but the seller never gets the carriage they want to charge so that's pointless anyway ?