So I’ve been making a Roblox game, and was wondering if a robux based marketplace would be against TOS. By a roblox based marketplace I mean a marketplace system where users can buy, sell and trade items using robux by buying another user’s (as an example) t-shirt. So would it be against TOS?
Only Premiums users should have ability to exchange their items so Roblox won’t authorize that.
I could make it so that only premium users can use the marketplace, if Roblox would allow it then.
I doubt it would. In the game PLS DONATE 💸 - Roblox, users are able to gain in game currency through purchasing gamepasses or T-shirts from other users. This game hasn’t been shut down for that 
I do however believe it to be noteworthy that if your “Items” are awarded randomly (through a spinningwheel or whatever), then you should by Roblox Creator Terms section f, clearly define the odds of winning, since the purchase of these items are related to robux.
Selling these items Off-platform is of course strictly forbidden.
Personally I think your game will be fine since there are plenty of instances of other games doing the same if not similar things.