With 2nd hand goods they have to fit description i.e. working. As a seller you are bound to that part of SOGA. If they develop a fault or whatever days or weeks later then you are not liable. (It'd be hard to argue this with PayPal though who allow cases to be opened upto 45 days after the sale).
Ebay/Paypal can add rules in addition to SOGA. It is their market.