Hello, I have a question. I bought a game in Poland on Allegro (a portal with auctions) - Mount and Blade 2. The seller gave me the offline backup installation files for this game. And hence the question is such a game legal?

No. It isn't. If it were legal, it would come in the form of a GOG key to redeem here. The offline backup installers are for personal use only and may not be sold.

Additionally, when the game is updated --- as it surely will, probably many times --- you will not get the updated installers, so you will be stuck with an old buggy version.

There are numerous greymarkets that sell GOG gift codes, which later down the line result in GOG removing the game from user accounts due to fraud. Trustworthy third-party sources of GOG games are Amazon Prime, Humble Bundle, legitimate boxed editions with a code inside, and not much else.

This is not legal and you should report the scammer to allegro. They're good with dealing with such cases and you will recover money.

Yes, I've reported this case to Allegro. Now I'm waiting for a response.