Ebay is not collecting GST on the sale of your items but on the fees that you pay for accessing ebay as a selling venue....the Final Value Fees on the item and the postage.

Everything you buy on ebay has GST included in the sale price but as the end buyer you cannot claim it back.  The only people affected by the GST on the sale price is the seller and then only if they are GST registered.   Ebay has nothing to do with this GST.