Transfer money between users?
I think the Ubercart funds module that is the best answer for your question; the module still has a development snapshot only, though. 

This will make you able to transfer funds through your drupal sites to different users. 


  Ubercart Funds implements a Site Funds Management System using Ubercart, It integrates features like Deposit Funds, Withdraw Funds, Transfer Funds..etc, which makes implementing most features of a Money site like Paypal or Skrill possible and easy using Drupal.