Hi Shane
Do you mean when you access your Account - Addresses - Primary postage address: Your main postage address for purchases- that your postcode has been changed here? If so sorry, no idea.
If you mean your postcode on the address label for the eBay purchases you have been receiving have been changed, it is possible that the sellers were using an automated process like Click and Send, which does not recognise some suburbs and postcodes as being aligned and suggests alternatives. Click and Send fpr example won't let you automatically print a label if it does not recognise the suburb and postcode as matching. Sometimes people put in an area rather than a suburb, or say a suburb within a city - like for example Kensington Sydney NSW - this would have to be corrected manually because the postcode (2033) would not match the suburb (Sydney). That's all I can think of.
