布小熊

I don't think regular jewelry stores will take 2nd hand jewelry. You could try a pawn shop (don po) but they pay the minimal to you.
If u want to sell Diamond jewelry, better to have the items appraised to find out what their current replacement/retail value. For 999 gold, some stores may pay as "raw material" or u can pay to get them redo.
If those are just Fashion jewelry, try selling it online but well I am not sure how safe it is. I've never sell mine.