So either a lot of you never played XI, or I'm in the minority in thinking that there isn't a huge issue with selling used gear. This was the norm in XI (excluding of course r/ex items), and while certain items did of course fluctuate in price over time, it didn't seem to be an issue. There were always players who needed the lower gear (new players, new jobs, level sync, etc), and so prices on that stuff stayed relatively stable. Personally speaking, I was much more inclined in XI to actually purchase gear every few levels because I knew I wouldn't be taking a huge hit to my pocket. As it is now, I try to make a conscious effort to switch it out every 5-10 levels. FATES don't require much as far as gear, and the lower-end materia is not worth enough to give back the money you've spent.

With all that said, I do think it would be an issue if they tried to implement the option to sell used gear as this point. With the gear being so readily available from NPCs and quests, there would probably need to be a lot of tweaking going on to make it worth the while. Also, different game, etc. But I do prefer the option to sell used gear. :P