Remove user account in Android 5.1
In my phone, I went settings->accounts, selected Google. There I have two e-mails  registered, I clicked on the first one, and a set of details opens.

On this screen you see on the top right corner a menu, that allows you to remove the desired account. It didn't wipe my Google's account - if you are afraid, you could just put it on flight mode before taking these actions...

In your case, however, instead of just deleting the account, I would do a factory reset before selling my phone (even if it is to a friend, due to personal stuff I have in it, like pictures and whatsapp messages)