Dopays is the worlds first open source paypal clone and has a very comprehensive administration system. Users can sign up and use their email address to make payments, and the script even includes a shop where users can buy or sell.
The idea being that members can buy credits to spend at other members sites, and the system can be adapted for use at sites other than the included shop.
Although the web site is now only a parked page, the last version of the script can be found at sourceforge, where they give details of their Twitter link, but if it's still active I don't know. Earlier versions of the script were known to have vulnerabilities, but there are no reports of their last version.
Also, the Dopays script has been copied, had it's template altered, and been re-packaged and sold by several different unscrupulous sellers.
The admin section is very detailed and allows the owner to set different rates or percentages per transaction, and has various integrations with other payment processors such as paypal itself, so that final transactions can relate to an actual paypal account. This way, sales through Dospays can be paid ulitmately into an admins paypal account, using the admins email address which is stored in Dospays.
Dospays also acts as an Escrow service so that admin can act as a middle man per transaction, and hold back payment until both parties are satisfied.
The installation should be fairly simple, but the files are written in php but appended .htm. This means a .htaccess file should tell apache to read htm as php, but this doesn't seem to work in later editions of apache. One solution is to find out why, and the other is to rename all files as php, but this would mean following through on every link on every page to make sure they are renamed to php.
If you have the time and want the script so badly, then it's available from soy=urceforge, simply search for 'Dopays', or I've included their last version here, together with other clones of the script. And if you get it going, please share!