There are just not enough gems in the game yet. I have beaten every level, paid the subscriber fee, and even simulated half of a million games. If I buy a few nice items for one class, then the other classes become unplayable. There should be a way to sell your gear (likely only getting back 90% of the gems for it or something). Even making it so that only subscribers can do it is fine by me (maybe once per subscription month). I feel like I should be able to change my gear around and try different classes if I am paying a subscription fee, as there are not a lot of levels yet. Perhaps when there are more levels you can take this feature away and players can try to buy every item in the game instead.
This is just my opinion, as I feel this is a very frustrating aspect of an otherwise fun game.