[rxlim]

After some research I'm now able to provide an answer to my own question.

The canvas was partitoned into blocks of 10x10 pixels that was white until sold. You would make a payment for X number of blocks through 2Checkout or PayPal. After transferring the money, you would send an e-mail to Alex Tew with the image, coordinates of wanted blocks, URL and TITLE/ALT text. He would then confirm the payment and upload your banner within 48 hours.

So the entire operation was manual with orders placed by e-mail.