I thought it would be more of a one on one trade. I send a message I need Utah. Someone posts they have Utah and they are willing to trade it for Kapro. I do not have Kapro but offer Spinosaurus. They accept and we trade Utah for Spino on a DNA to DNA basis until we reach the limit. So 500 for 500 (or an agreed on amount, 50, 100…) or a ratio for Common to Rare.
This way no one comes out an overall donor or over all requestor, to get you have to give. The two players come up with a mutual exchange. It could even be junk common for prime rare, it would depend on the agreement.
The exchange is out in the open so if anyone reneges on the agreement, it is known to all and no one will trade with them after that, self-regulating.
I originally thought the coins came from the person requesting, I did not know they were from the game. I would prefer if coins were not involved at all and it was just DNA for DNA between two specific users.