of course, 1%-3% doesn't make sense if you want to be in the control of the transaction between the freelancer and the client. But you don't have to be. The client should pay the freelancer as he see fits, in euros, pounds, bitcoin, shells or giant round stones.

If you can leave that aside and just charge the freelancer with that 3%, let's say. If this doesn't make sense, you can always set a minimum charge, like 5 euros or 3% of the transaction, which ever is bigger.