You may wish to look at ways to make gil outside of selling things to other players. Try treasure hunting if you have a high enough DoL, do daily beastmen quests, do your duty roulette every week, and do all your levequests. All of these are ways to generate gil. As far as selling items to other players, it's all a matter of finding something people want, but which isn't flooded in the market, and selling it. As an example, pudding flesh and certain tier IV materia were worth a lot of money before 2.1, and while the demand is still high, certain content has flooded the market with these, making them not worth as much anymore. The philosophy tome items also suffer from this, with quests that reward those items, treasure hunts rewarding them, and philosophy tomes being easier than ever to obtain via duty roulette and beastmen quests.