From PhpCOIN Documentation
Vendors are methods of payment that you accept, such as cash, cheques, and your third party billing solution. For example, Paypal is a vendor, as is "Money Order". The vendor record is used by phpCOIN when it is parsing PayLinks and when the user is returned to your site by the billing vendor after making a payment.
A Sample Vendor Record
Click on the Admin button, then in the 'Products' list click on 'Vendors'.
You can turn your vendor options on or off, and of course add, edit or delete vendors here. If you are only planning on accepting Paypal and not credit cards, be sure to turn off the credit card vendor option.
You are not going to add the actual options here, (ie: your PayPal address to receive payments) we will do that in Admin -> PayLinks.
Status: Whether or not this payment method is currently available for use Vendor Name: What we call this payment method (on listings) Buy Return Param Name: When we send a paylink to this vendor, they will send back a success/fail flag named this Buy Return Param Value: When we send a paylink to this vendor, they will send back a success/fail flag with the value of this for success/buy URL / Website: Where is this vendor's website located? Use IPN: Whether order return should apply a payment, or expect an incoming payment connection later and so ignore the buy parameter. Notes: For jogging your memory :)
Buy Return Param Name and Value
When a user is sent back to the orders page by a billing vendor, the incoming data from the billing vendor is scanned for a "buy" parameter. If found, the invoice is marked paid and a "thanks" message is displayed. If the "buy" parameter is not found or not set, the invoice is obviously not marked as paid, and a "sorry" message is displayed.
The "buy" parameter may be vendor specific, or it may be whatever you want ~ it depends on the vendor. For example, 2Checkout used to send a specifically named variable with a specific value to let us know the buy was successful. If the variable was not present, or did not have a specific value, phpCOIN assumes a fail and acts accordingly.
PayPal does not have a variable, so we can provide our own. We can call it "paypal" or "buy" or anything we want, and specify anything we want for the value, such as "good" or "yes" or "1". When we set the return URLs for PayPal we include a "good" buy parameter in the good link, and a not-good in the fail link. PayPal does not care what it is, it will just send the name/value back to us according to what the surfer did on their site. phpCOIN looks for the name/value and acts accordingly.
Other vendors may or may not have a specific variable, so read their documentation carefully.
URL / Website
This field is NOT used by phpCOIN ~ it is for your information only. For example, in this field the URL for PayPal would be simply http://www.PayPal.com and NOT the order or return URL that a paylink uses.