You purchased the goods, they agreed to replace faulty goods if a returns number was provided. They did not offer a money-back guarantee. As far as I know, you are obliged to accept the replacement goods. You could check the small-print to see if there is a cooling-off period, but you would still be liable for shipping costs incurred.
Everyone should be aware by now that they may be taking a slightly greater risk when buying from e-bay than when buying direct from a manufacturer. The only way that a business can make a profit at cut-throat prices is by selling in quantity, and they cannot afford the overheads of dedicated customer service help-lines.