Don't worry about the seller having several accounts. That is not the problem
Your problem is that neither ID is sending your purchases.
How long has it been since purchase?
I would allow 20 days from payment for arrival from North America.
If that has passed, go to the Resolution Centre at the bottom of this page and open an Item Not Recieved Dispute.
The first step is to contact the seller, but you have already done that.
If the seller cannot prove that the item has been delivered to your address, he will be forced to refund.
(Do NOT accept an offer of a replacement, money only and in full).
Because two IDs were used, you must open two Disputes. Don't fuss in the Dispute about the IDs, concentrate on the 'not received' part.
KISS