Presume that you are not a business seller?
If you are, then SOGA and the DSR may apply, but otherwise - as a private seller - you are not obliged to pay for return postage.
Distance Selling Regulations
The Consumer Protection (Distance Selling) Regulations 2000 generally apply to sales to consumers made by sellers acting in the course of a business, which have been made at a distance...
The Distance Selling Regulations apply to items purchased via Buy It Now listings and Second Chance Offers on eBay.co.uk. However, they don't apply to auction format listings on eBay.co.uk.
The Regulations also provide a period of seven working days after the date of receipt within which the consumer can cancel the contract (often referred to as the "cooling off" period) and get their money back, including the original postage and packing charges. The consumer can be asked to return the goods at their own expense, but only if the seller informed them of this requirement before the contract was made (otherwise, the seller is responsible for collecting the goods)
The cancellation rights do not apply to certain items including software, audio or video recordings that have been unsealed, perishable goods such as food or flowers, or items that have been made to order.

Sale of Goods Legislation
The most important piece of legislation relating to the sale of goods through eBay.co.uk is the Sale of Goods Act 1979 (as amended). The Act provides that wherever goods are bought they must “conform to contract”. This means that items sold through eBay by business sellers must be:
- of "satisfactory quality";
- "as described"; and
- "fit for purpose".
Therefore goods sold by business sellers must not be inherently faulty at the time of sale, must match any description given to them and, allowing for factors like price, they must be fit for their purpose, defect free, safe and durable.
Where the seller is a private individual, the goods must be “as described”. The goods are not legally required to be of “satisfactory quality” or “fit for purpose”.