Haven't seen anything like this, and I thought it might at least help someone :)
How to buy from Boomerang Rentals:Link
Click this link (takes you to the buying page, use the search to find whatever game you want to buy.) Click 'Buy Ex-Rental Now
' Then press on the checkout at the top of the page (it will appear once something is added to it.) Fill in your details and you've done it!
(The checkout page should look like the image below, but with your choice of game in it, not AC)