Most multiplayer games make it clear in their ToS that you don't actually own any of the items you acquire in the game. This is why you can't sue them if they choose to ban you or remove items from your character/account, etc., despite the fact that you could argue that results in a loss of assets or value (I.E. theft). I'm not going to lie, I haven't read GameFreak's ToS but it doubtless includes some sort of similar clause. So at the end of the day, it's generally up to the individual gaming company who holds the ToS whether or not selling items and other assets from its game for real world money or an equivalent is allowed; most do not allow this.
This leads to a whole rabbit hole of problems and conundrums related to selling virtual items. For example, gold from certain popular MMOs such as RuneScape and World of Warcraft holds a higher value on the black market than an equivalent minimum wage job in Venezuela, resulting in a huge number of Venezuelan gold farmers in those games farming and selling gold as a literal job to put food on the table. I don't generally support this kind of thing, but when it's literally a better alternative than finding a job in your country, it becomes pretty difficult to blame them.
