The game I've played that seemed to have the best economy was Tera. It worked well because it took all gear that dropped and turned it into a commodity through their enchanting system (Also the primary gold sink). This created a dynamic where low geared players would buy gear from higher geared players, and higher geared players would still have stuff to buy from lower geared players. Thus gold is able to circulate, instead of only trickling up. I will also note that after going F2P, there were no gold sellers because the cash shop sold trade-able cosmetics that allowed players to convert cash to in-game gold.
Explination of Tera's economy:
Gear was divided into tiers. Within each tier there were multiple sets of gear - fodder, low end enchant-able, high end enchant-able, and crafted. In order from best to worst, with only the crafted being bind on pickup.
Fodder was used as sacrificial gear to try to enchant. The higher the quality of your fodder, the better the odds of succeeding your enchantment. All players need fodder every time they upgrade a piece of gear and it provided an easy way for everybody to make money off the market, albeit in small-medium quantities.
Low-end enchanted gear would drop from bosses, and could be enchanted up to +12. All content for a tier is designed to be completed at +9 of the low end enchanted, and selling it could give you a moderate boost in money when a new tier comes out, and acts as entry level gear to the harder dungeons of the current tier. The price drops relatively fast down to a point that newer player can afford it easily.
High-end enchanted gear would drop only from the most difficult boss in the game, which had a lockout timer. This enforced scarcity, but the gear itself was still trade-able, so good players would be able to make a fortune.
Crafted gear was the absolute best, and the materials only dropped of the most difficult boss, but since it was bind on pickup, it didn't play much role in the economy.