I will be glad to donate for this extension.
Easy to customize, and I am not a expert coder.
Have all that you can need for user message services and also more.
I am using it in a ecommerce where single users can add and sell object and the site owner can manage and check any transation, UddeIM is perfect for enable communications between all the users!!
Is simple and working fine, easy to customize as you wish, very flexible and can be extended in a while.
Plug In to enable online payment are available also for J2.5 under a very low price (past week on JED was result only for J1.5, I apologize for bad review).
If you need a simple shop, this is the best solution: simple, easy to manage and customize, FREE.