I'm assuming you have sent them -

Item not as described negates change of mind returns, which is what the buyer, according to you, is claiming. eBay will back the buyer 100% if the item isn't as described. If you feel that the lack of a region code is not relevant, by all means fight it. Understanding that eBay will probably refund the buyer at your expense, let the buyer keep the DVDs and give you an unresolved defect.

If you want the DVDs back, you will have to provide the buyer with return postage, so you will be out 2 lots of postage.

Ps: Put ALL relevant information on your listings, whether you consider them relevant or not.

Pps. eBay don't care whether you are a professional seller or a claimed house clearer. The only way you would get any special dispensation would be to be a big box retailer.

Ppps. You must have a big house.