Back in the day you would use Auctiva as your 3rd party checkout. Adding your Authorize.net was easy to add to the checkout simply put in your API and Login key which you would find in your account in A.net. Buyers were directed to Auctiva checkout and had to pay by credit card, you could check off Paypal so they were forced to use credit cards that you took as payment
As of 2010 if you have a merchants account and Authorize.net it needs to be through ebays checkout, you need to go to your site preferences in myebay, give them your API key and login and run a test charge of $1.00 thats it, you dont need to send ebay your merchants account information any longer, it is recommended that the account be aged a bit and in good standing using Paypal for about 90 days before you start taking in payments through a merchant account so you dont waste a login key.
I have had numerous authorize.net accounts attached to ebay accounts feeding a single merchant bank. If they get suspended just close the a.net, and open a new one however that can get expensive but worth it if selling with a merchants account.
Setting up a payment gateway for your Internet merchant account
If you already accept credit and debit cards through an Internet merchant account, you can use checkout to accept these types of payments once you provide us with your payment gateway account information. Learn more about payment gateways and how they work.
To set up your payment gateway on eBay checkout:
Click My eBay at the top of most eBay pages. You may need to sign in.
Place your pointer over the Account tab, and then click Site Preferences.
In the Selling Preferences section, click the Show link next to Payment from buyers.
Click the Edit link.
On the Payment Preferences page, select Accept credit or debit card payments to your Internet merchant account.
Next, you'll need to enter your payment gateway information.
Note: If you need help finding any of this information, please contact your payment gateway provider.
To enter your payment gateway information:
Choose your payment gateway provider from the drop-down menu at the top of the page.
Enter your Merchant login (sometimes called "gateway login" or "API login ID"). This is the ID that identifies your account at the gateway for transaction processing.
Note: This ID is usually different from the user ID you use to log into your gateway's website.
Enter your Password (sometimes called the "password" or "transaction key").
Note: Usually, this is a password that was generated by the payment gateway for you, not a password that you created.
Depending on the payment gateway you use, you may need to enter a Vendor ID or User ID.
Depending on the payment gateway you use, you may need to enter a Partner ID.
Select the credit cards you'll accept through this payment gateway.
Select your merchant preferences, such as whether you'll require buyer's credit cards to pass Address Verification (AVS) and Card Security Code (CSX/CVV) checks.
Enter the credit card information for a credit card you accept and that's accepted by your gateway. Note: Your credit card won't be charged. We use this information to test that everything works correctly.
Review your information and confirm it is correct.
Click the Submit button.
Once you have your payment gateway account information setup with checkout and enabled in your preferences, buyers have the option of paying you online by credit card.
The fine print
Payflow payment gateway, which sets up a secure connection between eBay and PayPal, is governed by PayPal's user agreement and privacy policies, but isn't linked with your PayPal account. You don't need to have a PayPal account to use Payflow on eBay.
authorize.net and auctiva 
