I think it's like PayPal, where you see that payments can be made using a credit card or your PayPal account. And, like Facebook and Google, where you can "sign in" to all sorts of websites by using one of those ids that can be verified. They allow you to log in with a username from a common function that already shares info. You also already know that FB and Google and even PayPal will market to you even after death, though. You likely did nothing wrong as long as you confirmed the little lock was secure at the top and the website was the real one. That doesn't prevent them from being A**h**l*s which is why I don't use any of these functions or have those accounts. I bought a snowblower off of FB marketplace by asking my mother to contact the seller (she uses FB to link to lots of other family) and then I tried to make a FB account for my cat so that I could sell a snowblower. They let me have it a couple of days, then they wanted a phone #. My cat has no phone. Oh well.