Assuming you paid with Paypal, open a Paypal dispute. Don't bother with eBay.
Contact the sellers as described and if they will not accept the return (at their cost) or a genuine replacement then explain (politely) that you will open a Paypal dispute and leave them negative eBay feedback. Give them a period of time to respond - say 7 days (make sure you don't wait till more than 45 days from the date of payment though or it's too late).
Always contact them using the "contact seller" option in the listing - that way all the mails are logged through eBay in case of any problems....
If you don't get it sorted, open a Paypal dispute (login to your Paypal account and go to Resolution Centre) - it costs nothing.
Open a new dispute, saying the item is not as described.
Paypal then contact the seller saying a dispute has been opened and give them 7 days to respond. If they don't answer within 7 days, they find in your favour and you automatically get a full refund through Paypal. Usually the seller will make you some kind of offer - you can either choose to accept it or "escalate" the dispute to a claim.
It's highly unlikely that, assuming the listing states they are genuine mats and they are not, that Paypal will do anything other than find in your favour. If the seller does not offer to pay the return postage either, you don't have to return them.
If it's a genuine mistake on their part then fair enough - if you feel they've deliberately mislead you and they are selling / have sold other similar fake items, report them to eBay for selling conterfeit stuff. eBay are quite strict on that kind of thing and will act. In those circumstances, it's also worth leaving negative feedback on eBay to warn other customers.
More info on the Paypal process here:
https://www.paypal.com/uk/cgi-bin/we...nIdentifyIssue