You will unlock Exotic Merchant at level 30. Here you can sell items and scrolls and receive Exotic Coins in return. With these coins you can buy permanent upgrades. Also at level 65, you will unlock Emblems. From this point on, you receive emblems through missions and you can buy emblems with gems. With these emblems you can buy Epic Chests at the exotic merchant.
Back to exotic coins. You want to sell pretty much everything with the exception of (certain) gold items. Either you use all gold items for quicker prestiges and meteorites. Or you only keep gold pouches (10 coins) and buckets (35 coins) for the sole purpose of completing the daily quest. Use 4 gold pouches, 40 coins spend and you receive 3 scrolls in return, worth 240 coins. Profit of 200 coins.
As for exotic upgrades. Your first priority is Prestigious. Yes, where as Prestigious in Firestone Research is not that good because it’s low percentage and linear. Inside Exotic Merchant, Prestigious is higher percentage and multiplicative. But it has a level requirement. You need to be at least level 50 to buy the first level, and that level requirement goes up by 2 for every purchase you do. Second priority is Raining Gold. After those 2 are maxed, you go raise attributes. Depending on the build, you can go many ways. With a glass cannon build, go for Hero DMG. Then you must decide, All Attributes to support all kinds of heroes, or ready yourself for a mana spellcaster party and level up Magic Spells. All up to you.