You are missing one simple thing here. All the best guns and armor comes from PvE via the raid, nightfall, or heroic (as in strange coins to use with xur). Everything the vendors sell is crap compared to raid and exotic weapons and armor. You really want to run around in circles for hours for a few marks? It would take like all day just to get enough marks for leg armor, much less a helmet or a gun. I don't get it, I've hardly ever used the mat exchange, that is probably why they removed it, because most people never used it. I used all my materials leveling up raid guns. And that was too much farming for me. I would much rather buy my materials. Never had any left over to trade for marks. Not to mention I never had any need for much of anything from the vendors. Usually just bought stuff to break down into shards to level raid and exotic armor.