I'm having a similar problem.
Client is using venmo as a bank. Taking deposits and making payments from it, most of which do not go through her bank accounts at all. Although sometimes she transfers the funds to her bank account, and sometimes a venmo payment comes out of a bank account, but most of the time the in and out is in venmo alone  I set it up in COA, but am perplexed as how to keep track, and balancing venmo seems impossible. 

I was thinking of assigning an account number for transfer, for when it shows up in a bank account AND venmo, to show the flow-through, but now wonder if the actual cost should be itemized in venmo or the bank. Argh.