I've never even had anything rare lol.
Theres pretty much 2 ways to get "rare" items.
You can farm Stat Potions and trade them for items, although its probably smarter for you to drink them yourself.
Or once you are confident with your character, doing a lot of damage to super bosses (like the cubegod, skull shrine, pentaract, or the mad god oryx)
Now, once your decked out in the best gear you can get from doing that, its time to attempt to get into the Wine Cellar. It can be opened by a rare key dropped by dungeon bosses after you kill Oryx, and it opens up another dungeon where you fight him again, but this time he drops the best gear in the game.
Also, RotMG has a lot of real life trading in it, and the devs dont care.
Here is what people will buy if you want to make some money off Realm
Potion of Defence = ~$1
(dropped by gods in the normal realm)
Amulet of Resurrection = $80
(sold for 14500 fame in the nexus)
Anything dropped by Oryx2 in the Winecellar = ~$50
High Level Unique items from dungeons = $20-$70