Hi cazzlewhacker,
The use of Auctiva checkout is a default setting on your account and an auction will go through Auctiva checkout if the following are true: 1) it was posted via our site. 2)At the time it was posted, you had checkout enabled for your account. Otherwise they will not.
If you don't wish to use Auctiva checkout, it can be disabled at Auctiva -> My Account -> Checkout Settings. The screen shots of the checkout process are also available on this page so you should definitely check them out.
-Mike