Here's ebays rules
Under the Distance Selling Regulations, buyers have a period of 7 working days after the date of delivery within which they can cancel the contract (often referred to as the "cooling off" period) and get their money back, including the original postage and packing charges. You must refund the original delivery charges. However, you are permitted to require the buyer to pay for the cost of returning the item, but only if you clearly inform the buyer of this before the contract is made.
ALSO
Do I have to refund a buyer if the item is faulty or damaged?
of ‘satisfactory quality’;
‘as described’; and
‘fit for purpose’.
If an item you sell doesn't conform to these criteria, the buyer has the right to request money back within a "reasonable time". What is considered a "reasonable time" varies from item to item. For example, a pair of skis is unlikely to be used straight away, so a "reasonable time" is likely to be longer than for an item normally used on a daily basis. Other factors may also affect what is defined as a "reasonable time" for specific items.
In addition, the Sale and Supply of Goods to Consumers Regulations 2002 gives consumers a right to a repair or replacement where goods turn out to be faulty.