Diamond wasnt have high resale value, not common for people to sell it, maybe you can try pawn shop ...

Another option was, normally the original shop if have trade in service, i remember is 70/80% base on the original amount, if you have any friend or family member plan to buy diamond ring then maybe you can offer them to trade in yours, then you can get the 70/80% back ...

Ohh i see, never know have such T&C .... wish you can find a higher selling place ...