I do middleman like this:

1. Both of traders do have to add me.
- I will never join any chat before talking to both traders [Scam protection]
2. I will talk to bot traders, set my fee + we will proceed to transactions.
3. Trader which has ITEM sends item to my invertory.
4. Trader which has MONEY sends money to me.
There are more options for which i offer my middleman service.
- PaySafeCard
- Bank Transfer
- Skrill MoneyBookers
!!! PAYPAL is gonna be a littlebit different. I'm just gonna hold CS:GO item and trader which has MONEY will send PayPal transaction right to trader with ITEM! [I'm doing it like this, so i dont have to face any refunds'n stuff.]
5. I will proceed money to the owner of the ITEM.
6. I will release the ITEM and i will take my fee.

My fee is 3% of the ITEM value.