Well what you can farm at 50 will depend on your server. There are a few items crafters need that only drop from mobs in the open world. However, if your server is like mine you won't make much from doing that. Your best bet is to run some dungeons say Darkhold and need and greed the gear and sell it on the market. I just sold one piece for 55k today. Now that is worth your time. The best you could hope for farming the mobs I mentioned is 5-10k and it is a grind to get a stack. The other option is converting weapons and armour into materia after spirit binding them, some materia sells for a very good profit as you can tell from the purchases for your relic. The only other advice I can offer is find items not on the market board and supply them to the market, either through crafting or from NPC purchases.