There are two factors in determining the value of a card:
Rarity ( Common cards will appear more than Legendary cards )
Personal worth ( Card A has a better effect than Card B which has a higher rarity. Card A is better than Card B in specific decks giving it a higher value to some users )
Are we allowed to let players trade Cards that are rare in the system ( e.g. are the hardest to get out of packs ) with cards that are of high value depending on if you’re using a specific type of deck?
In the game you’ll be able to buy money which you can buy packs, I believe this plays a big part in deciding if this is allowed or not.
Is there something in the ToS or community guidelines or something that you’re concerned about in particular? I don’t recall any rules at all regarding trading things in a game (besides trading in-game items for real roblox limiteds, which is not allowed).
Have you read something somewhere that gave you the impression it wasn’t allowed?
There are many games in which similar ideas are incorporated, the main ones being those that involve pets. In these games pet values are also determined by rarity, worth, and in some cases power, and they are also bought using in-game currency.
I think as long as you’re not allowing the trading of your cards with limiteds or robux itself, you will be fine.
Is there a such trade where personal opinion on the value of items doesn’t come into play? I feel like everything has a sort of sentimental value to someone. So if you’re trying to trade that would always come into play. Even if you outlined the exact value of each trade as Roblox does with their trading system, whether or not the user decides to accept and reject is still solely upon them.