Pretty sure you still do this manually and I'm not sure if this one is already included in your pending upgrades for your Middleman Service but i still want try and put this. Whenever someone purchased a product via Middleman Checkout Link and completed the payment process an alert/notification will be sent to the seller (probably same execution on how it notifies us when you input our Middleman Balance in our account). What it does is it will prevent multiple purchases of the same product and it will help the seller to focus their attention on advertising their other products while the Buyer Verification is still in-progress.
This feature will be available in the Middleman 3.0 which is a completely new middleman system which will co-exist with the current middleman system. The new middleman system will be a fully organized middleman system with a backend. For the old middleman system, this feature can't really be done.
We will however be releasing new notification settings in the upcoming days to hopefully offset some of this as we want to improve alerts and also add additional ways for members to contact other members.
Talking to @Middleman, they may push for this to come out during the summer. It depends on several factors right now. Basically it's a more automated middleman system that will either replace or will be another option for sellers.