TL;DR: if I can both buy [i]and[/i] sell materials at faction vendors, I can game the reputation/materials/marks economy in a bunch of new ways. At least one of these, and possibly all of them, probably caused balance concerns on Bungie's end of things. *** I'll start with an example. Suppose I have 50 Vanguard marks and 200 Crucible marks, plus 210 spinmetal. (Coincidentally, this is exactly what I have!) Bungie has now added the ability to buy materials from the faction vendors at a cost of 10 marks per 20 mats. So let's just go ahead and convert all 250 marks to more spinmetal. That's a total of 710 spinmetal in my inventory, now. And if I could turn around and sell that spinmetal back to the vendor, I would gain 70 marks (700/50 * 5), 3500 glimmer (700/50 * 250), and 350 reputation (700/50 * 25)--in addition to 10 leftover spinmetal. Now I have 70 marks again, and I can turn around and buy mats with them--140 of them. Now I have a total of 150 spinmetal, which I can sell those back to the same vendor for another 15 marks, 750 glimmer, and 75 reputation. So with about twenty minutes of effort and my initial stack of stuff, I've traded 250 marks down to just 15 and lost my 210 materials--but I've gained 425 reputation (not including the Nightfall bonus). And I can probably do that every week, since in addition to farming I'm now getting 26 materials per day from patrols and daily story missions. (Of course they won't all be the [i]same [/i]material, but that's not critically important.) And that reputation gain is on top of whatever existing reputation gain I'm picking up from bounties. I can do several new things in this system--that is, the one Bungie prevented by removing the ability to sell materials for reputation: (1) Trade materials for reputation with any faction (plus some glimmer). I could do this before the patch, of course, but the effect would be multiplied by 20% even without investing any additional marks. (2) Exchange Crucible marks for Vanguard marks or vice-versa, at a 5:1 rate (plus a bit of reputation and glimmer). This has never been possible before. (3) Exchange one material for another, at a 5:1 exchange rate (plus a bit of reputation and glimmer). This has also never been possible before. (4) Spend marks to gain reputation, at an effective rate of 40 marks per 25 rep. This has never been possible before.