you stated what was wrong with it, its like buying anything second hand there is no receipt or garentee, basically i'd personally go tell them to go f**k themselfs, no matter what threat they use there is absolutly nothing they can do, ebay wont be interested, paypal wont be interested, i very much doubt small claims court would be interested, and even then it's a farce, you dont turn up nothing happens basically and it costs more money than they will be claiming for, which means basically it isnt a option anyway
Change your number or tell them your not interested and to have a merry christmas, tell them to buy a cheap mower with the scrap value cash :D