When you said that you accepted having to pay return postage, did you mean that you have been in discussion with the seller via eBay Messages and have actually agreed to this? If this is indeed the case then you have done the wrong thing. The correct course of action would be to open an Item Not As Described case against the seller, stating that you want to return the item for a full refund. The seller would then have to provide you with an eBay Returns Label so that you could return the item for a refund.
Once the eBay Returns Label has been issued print it off, affix it to the package that you are going to post the item back in, and then post it back to the seller. eBay Returns Labels all have tracked postage, so once the tracking confirms successful or attempted delivery back to the seller you should get your money back.
In the event that the seller proves to be awkward make sure that you escalate the case to eBay as soon as you get the chance to do so. (If I remember correctly this is by day four of the case.) If eBay have to step in and force a refund you should end up with your money being refunded in its entirety, and under those circumstances you may not even be required to post the item back to the seller.
