Tax 10% dividend from the pot for the old players

-1.

The idea doesn't make sense for several reasons:

• It is impractical and unnecessarily tedious to implement.

• Once the buyer has paid the purchase price, ownership of the Pokémon is transferred to them. They are then free to do whatever they wish with it.
  

It may be frustrating when someone purchases a Pokémon and immediately resells it for a profit, but that's an inherent risk when selling anything. Once you've sold something, what the buyer chooses to do with it is ultimately out of your control.

If you're concerned about this happening, do your research and make sure you're comfortable with the price before agreeing to sell.

Edited 22 minutes ago22 min by Dos3d