Bought used MacBook Pro, can’t log in

If you can't get past the login screen, did the seller tell you that he set it up with an admin password?

My suggestion is that you contact the seller and either have him tell you what the password is and also if he used the proper steps outlined in this article preparing the device for sale or I'd demand a refund.

https://support.apple.com/en-us/HT201065

(Note: #6 - reformat/erase and reinstall the original OS - is very important).

What model? It might be possible to just restart from scratch if it's capable of Internet Recovery. If the drive is dead you might need a new one.

About macOS Recovery - Apple Support

Reformat (erase) the drive using Disk Utility and then Reinstall MacOS.

Hahubb wrote:

Bought on eBay, trying to get that. Going to Apple Store tomorrow. If I can’t figure out or get answer. Apple support says firmware is locked. Can’t do the Command P or S or any of that.

No command-R either? Apple can theoretically unlock a firmware password, but they might want to see an original proof of purchase. The big worry about bypassing a firmware lock is that it might be stolen if the seller didn't disable the password first.

Since this is a used Mac, check here > Changing ownership of a used Mac | Communities

Good luck tomorrow; FWIW, 90% (so I've read) of electronics and software sold on ebay comes with a problem (be that illegally obtained software/OS, re-using registration keys dozens of times, selling Macs with pirated software, and the list goes on). I've tried buying a Mac there twice: the first time, the seller claimed it was new when it obviously was not; the second time it had illegally obtained Mac OS (and both sellers had a 100% rating). I no longer believe anything I read/see there.