@Iron_Guard8I remember someone compared the market here to the way buying cosmetics work in free to play battle royals games. Since people actually spent money to buy the game, I think it is only fair if the costumes were at least more accessible.
Although the role of the Reward-o-tron is being wasted, my solution would be to make Rainbow stars easier to obtain. Not just by getting more through challenges and prize maps, but by also being able to trade in coins/currency for Rainbow stars.
Or make cosmetics available to buy with coin prices or rainbow star prices separately