Summarize:

You can use any web browser on your Windows or Mac computer. If you aren't signed in automatically, click "Sign in" in the top-left corner of the page and log into your eBay account, or click "Register" to create a new account. Once logged in, your name is in the top-left corner of the webpage. This opens a drop-down menu with some account options. It's just below your profile picture in the drop-down menu in the top-left corner. It's in the sidebar to the left in the center. It's in the seller preferences section. The seller preferences section isn't available until after you list your first item and have created a Seller Account. It's across from "Payment preferences". It's near the bottom of the Payment Preferences page. It's at the bottom of the Payment Preferences page. This allows you to accept credit and debit cards to your merchant account.
Go to https://www.ebay.com in a web browser. Hover the mouse cursor over your name. Click Account Settings. Click Site preferences. Click Show next to "Payment from buyers". Click Edit. Click the checkbox next to "Accept credit and debit card payments". Click Save.