The coins are absolutely yours to spend how you want. If someone said they were buying three of any one item, I would question the need now-a-days between the Companion app transfer and the (much more convenient) community built Chrome Destiny Item Manager. Would say the same thing if they were buying three Gjallarhorns also. HoW is right around the corner, and you know we'll be seeing Xur selling HoW exotics as soon as it drops. At full loadouts, that is 13x3+23=63 coins (assuming all three exotic armor and an exotic primary). Considering you only have 27 gaurenteed exotic per week that could be a problem for some people (not including Exotic shards to upgrade "old" exotics, and Heavy Weapon Ammo packs). Personally I'd recommend saving coins for the future, but hey, I'm on a slower track and don't have the pile of 700+ coins so many seem to have. You spend your coins any way you want. If he does sell it, I hope you get as many as you want and I hope it lives up to your expectations.

This^