My Macbook Pro just locked for no reason.

Have a look here too.

I'm basically screwed. I was actually able to contact the seller last night and found him in an old conversation, I told him the problem and he said he doesn't have the Macbook linked to his iCloud, I tried with one possible PIN he provided and it didn't work. From what I understand he is as well not the first owner.

I suppose that's illegal. Most probably, you have set that yourself, as you've factory reseted the Mac few years ago.

What's illegal? selling something? How is a regular person supposed to know that when you purchase a device it's linked to iCloud, common sense is that once you factory reset and remove your account from the computer or any device (Windows, Android, Linux) it should be ready to sell.

log on to your Apple-ID page on Apple.com from another device. if your (now-locked) Mac is there, if you ask it very nicely, it should divulge the pin required to be entered at the device to unlock.

I don't have another Apple device.

A very careful reading of the "Find my Mac" article suggests that, but the description provided is very hazy, possibly by design.

Mark a device as lost in Find My on Mac - Apple Support

see the section labeled, 'Unlock a Mac'

Well that basically took me back to https://www.icloud.com/find/

And like I said before, the Macbook is not there listed, I didn't even knew about having to link an Apple device to this iCloud thing was necessary or removing if you are the previous owner when selling, seems ridiculous to be honest, when the seller gave it to me it was reset, I went through the setup process and there was no other User, it makes no sense it's not in my iCloud or worst if it is still in someone else iCloud

I contacted the previous seller, it seems like he doesn't have it either in https://www.icloud.com/find/

Well, this made me realize.

I can actually see a Macbook Air I purchased some time ago that I sold in Facebook Market completely factory reset.

I can Lock it from here!? That's just crazy...

That makes me realize that the guy who sold me it possibly locked it, maybe by mistake or trolling.

If my previous assumption is correct...

I just cannot believe it, I understand this is a way you have to lock a stolen or lost device so people cannot use it, but if it was factory reset and a new account was set in isn't it ridiculous? I hope this is not the case...