BTW, I contacted the guy who sold me the phone by mail with attached photo (seems I cant attach it here) and so far he hasnt replied.

If he legit stole this from Amazon, he ain’t returning your money. If he’s allowing you to get your money back (I am still suspicious), then take it.

Have you set your social media profiles for friends only or private?

You should be able to change it so only people you know can view your profile and comment on pictures. Its pretty easy these days to track someone by name or even phone number on social media. Making your accounts private or even viewable by friends of friends harder.

(he kept on coming back and commenting on my facebook posts from numerous fake profiles

I don’t think someone commenting on photos is hacking into your account that makes zero sense. The only reason to hack into someones account is gain access to photos or use it like randsomware and demand money for the return of your profile back. As mentioned above change your security settings and only allow friends or keep your profile private.

I would as i said go through all your profiles and set every single setting to private and DO NOT add any or allow followers from people you do not know.

she received an email from Samsung which states that another Gmail account (her classmate’s Gmail) is logged into her phone.

You usually get those emails a few minutes after logging into a new device, not “A long time ago”.

The email should state generally location, time and date.

Someone took my device did he install anything suspicious?

Who?? Are we talking about someone at a mobile repair kiosk, a buddy of yours, or a jealous BF?

The first thing you should check is anything out of order or an app opened that shouldn’t be (banking apps, facebook, twitter, instagram, files app).

As another person stated, you can always backup your data, erase the phone and start from scratch if you are super paranoid.