If you mean that when you check out of eBay and select PayPal, you are sent directly to a specific PayPal account without entering username or password and you do not want that to happen, sign in to that PayPal account outside of eBay and use the Gear icon to go to Settings and choose Security, then check the "Manage your Logins" and "Auto Login" sections and either delete those entries or select "Log out of all devices".
If you are unable to sign in to that PayPal account directly, try using a different browser (or a private or incognito browser window) to see if that gives you the option to select a PayPal account.
If you mean that when you check out of eBay and select PayPal, you are not able to choose a specific payment method in your PayPal wallet but one is selected for you, in that case go to your eBay Payments page in your eBay account and delete the entry for PayPal that appears there.
