There are a few options. First off, if he is selling a specific exotic u want, grab that. If not, you can divvy up your remaining coins between heavy synth and three of coins. Until u hit 390, rank packages are a great way to raise your light. It takes 100 heavy synth for a faction rank which is just over 33 strange coins. This is probably the easiest way to raise faction rank. It's also always good to keep a healthy stock of three of coins. I usually get an exotic roughly 1 in 7. Sometimes more and sometimes less. I've had less luck using them on strikes and general pve lately. Done alright either in crucible or farming the iron lords in the last RoI mission. Since I got over 390, I check the stock (pretty sure I have everything he can sell now though) and then dump the rest on three of coins. It's safe enough to use all of your coins each week as they are fairly easy to come by these days.