Direct Payments between your website users (bitcoin)
By Bitcoind or API or ..
userA pay to userB or .. , userB pay to userA or B or.., etc
Only need to "payment script".
On the site, a user must pay to the other Members. The user clicks on pay. (Previously designed)
Site sends a bitcoin Address to the "payment script".
the user enters the payment page. (in the script)
script returns results after After payment of user. (paid or unpaid)
Proof of payment (required 2 or more-confirmation of payment transactions)
All will be in automatic mode. + Security