If you have not had a satisfactory response from the seller, your options really depend on how you paid, as well as how long ago.
If you paid by PayPal and it is within 45 days from the date of payment, you can lodge a dispute (which you then can escalate to a claim within 20 days if there is still no satisfactory resolution, note that you should never close a dispute if the seller asks you for one reason or another - only if you have your item or a refund). The same 45 day limit applies to disputes via Paymate.
If you paid by cash, bank deposit, cheque or money order, there's very little that can get your money back, but you can report the seller to the online fraud department.
If your payment was funded by a credit card, you may be able to initiate a chargeback (even if you paid via PayPal), you would need to speak to your card provider about whether it's possible, as they all have different limitations.
