eBay's money back guarantee doesn't include collection. The seller is required to send a prepaid return label. If it doesn't cover the weight you need to contact eBay customer support.
Seller collection of returns is risky for the buyer as the buyer will have no tracking record, as eBay requires to confirm items have been returned.
Petrol lawn mowers are tricky. It will need a courier. They will probably require all petrol to have been drained. You may also need to drain the oil, unless the courier can undertake to carry it the right way up. (Mine was delivered upside-down, and everything in the box was flooded with oil!)