What are crates? As in, what do they give you/offer to the players progression?
If they give some kind of cosmetic, maybe make crates obtainable through robux and also playing the game, and make the cosmetics they give you tradable
Personally, I think letting players trade/sell items for robux would just make them invaluable.
Would rare items even be rare if you could just buy one?
In a game like murder mystery 2, you can get a godly (highest rarity) weapon by either unboxing it in a crate or trading for it.
These godly items are rare because they’re a really low chance drop from crates (obviously), and also because its not easy to trade for them. You’d need to trade your way up and get very lucky
I think people being able to trade them for robux would make them too easy to obtain, even if it would cost a lot.