Anything he sells that you don't have and can afford, remember if that weapon or armour piece doesn't look like it suits you now it may in a weeks time or different activity. The perks requirements change all the time, by that I mean I never purchased a helmet that replenished health with orbs because I didn't need it then crota hard mode came out and I needed one so I had to wait for a drop or xur again. Buy everything you can afford.