Bought Used Mac Mini - Apple ID Issue

Hi all! Just bought a Mac mini off of eBay and got it hooked up. When it came to log in to my Apple ID, it says my password was incorrect (it wasn’t, verified on a separate device) and my account was locked. It said to go to iforgot.apple.com. I checked before I went there as this didn’t look legit and it’s not! I logged in with a fake name and no Apple ID, but now I’m paranoid about using this thing. I’ve reset all settings and erased it (it’s going through now) but obviously something is embedded that shouldn’t be there. Any advice?