Jewellery, particularly diamond pieces are one of the very few articles that after you have finished with them you can take them back to a store and get some money back. You don't buy a coat, wear it for years then take it back and say can you work out the value please and give me some money?
Don't forget, whatever the retailer gives you will be the lowest price - he is gambling with his own money, the item may have to be broken up and the stone reset into a new more saleable item. All this costs money, and something is only worth what someone else is prepared to pay for it.