To allow your friend/spouse/ employee access to your eBay account, tell her your password. She can now do anything on eBay you can do, including responding to questions from customers and leaving feedback.
To prevent your friend/spouse/employee from having access to your Paypal account, don't tell her your password. And make sure it is very different from your eBay password. She will be unable to do anything with your PP account, including printing out shipping labels , doing bank transfers and making refunds.
