My experience too is you can get up to about half on secondary market. It is best to see if the original jeweler has a trade in policy. Some places only make you spend $1 more and some make you spend double. I would contact them first to see. Since he is selling to buy new he might get a better deal that way. Also, does he want to reset the diamond he has into a ring for the new girlfriend? Maybe get a gorgeous double halo?
I'd be ****ed if I found out my bf gave my stone to a previous chick. Its one thing to be used, its another to be recycled by the same guy to different girls.
