Is this a good deal?

Offer $375 and try for no more than $400.

Hell no, look elsewhere

200$ all day.. going to need a 1x drivetrain and a dropper post asap and maybe tubeless set up and tires (if they are dry-rotting)

There’s a fine line between you making a decision and asking us to make one for you lol. For the money that bike looks on par with what sells in that price range. It’s okay. Will you get a lot more by spending 2x the money l? Maybe. As the bike is about 1/2 the price of a new bike of its class. You can ride it casually most anywhere.

The important things you should be considering is
1) where are you going to ride?
2) how long will you be riding?
3) how often are you going to ride

And to shed more light there are multiple types of so called mountain bikes. Hardtail (the one you showed us) no back suspension. Then there’s enduro and cross country and downhill. Then there’s looks like a mountain bike but used to get around town. Then there’s gravel bikes some with straight bars others with drop bars like road bikes. They have wider tires like 1.95 width instead of 2.25. And you can run these thinner tires on mountain bikes around town.

Then there used to 26” diameter wheels later they went 27.5” then finally 29” wheels. That all comes into play with costs of used bikes.

So just really think about what you want to do. Maybe look at every Facebook market place add for a week just to see what’s selling at various price points.