You seem to have gone about this the wrong way. Return requests are made through eBay, not by you trying to contact the seller. It would also have helped to tell us when it was delivered, and your seller's returns policy, as your rights depend upon this.
If it was a UK business seller you have a right to a change of mind return - probably 14 days, but check the listing. (The time limit applies to the date by which you need to request the return). To submit a formal return request through eBay, see: https://www.ebay.co.uk/help/buying/returns-refunds/return-item-refund?id=4041
If you request a return within the time limit and the seller refuses to honour their returns policy you can contact eBay, who can change the return request into a money back guarantee case. The seller cannot refuse this.
You will obviously need to disassemble the desk, and if possible return it in the same packaging, although the seller cannot insist on this. You will be responsible for the cost of returnng it. If the desk is not returned in the condition supplied, as a business seller the seller can ask eBay to reduce the sum refunded to compensate for this.
I hope that you're still within the time limit to request a return. If not, I'm afraid there's probably no way to get a refund for a buyer remorse return.
