Hi cama music,
Welcome to Auctiva. The only listings that will use Auctiva Checkout are those that were posted through your Auctiva account while the Auctiva Checkout setting was enabled, so your listings that were already active before you signed up for Auctiva will not use the Auctiva Checkout system. If you never posted any listings through Auctiva prior to disabling the Auctiva Checkout settings, none of your listings should be using Auctiva Checkout.
The primary functional difference between the Auctiva Checkout system and the eBay Checkout system is that, if you have an Authorize.net merchant account, you can integrate it into the Auctiva Checkout system to allow your buyer to pay via credit card without using Paypal. You can see screen shots of the Auctiva Checkout system by clicking the "Checkout Settings" link under the "My Account" tab, and then clicking the "View Screen Shots" link on the ensuing page.
Any additional questions or concerns? Please feel free to contact our customer support team using the appropriate link on our help page: http://www.auctiva.com/free-auction-software-help/help.aspx
-Mike