As far as I know, the most trusted exchange that offers shorting is Poloniex. If you haven't used it before, I recommend setting up an account there, as it is very easy. Correct me if I'm wrong, but Poloniex has the largest volume on all coins by far. Don't know what you mean by instruments, mind specifying a bit?
Are you sure, we can do sell first and buy later kind of trades in poloniex ?
I am sorry I am not finding any that kind of options with them. Are you denoting "STOP-LIMIT" orders ? I guess this is a functionality for order values other than current market value. If you are finding time to explain on how to do shorting on poloniex exchange with screen shot, it would be highly useful for numerous traders here.