The computer appears to be pre-owned or perhaps was a demo model, and the store
should exchange it for one you have to setup and use yourself. This means it would
have a startup screen where you'd be introduced to your new computer and then enter
your personal information, which later is used for security purposes and your account
with Apple, an AppleID can be created if you did not previously have one, and buy a
new computer product. Before you could use the App Store, iTunes Store, & so on,
the unit has to be correctly identified to your Apple ID.
If the store (which I have to assume is an independent reseller of Apple products; though
may be an 'authorized' reseller, is not an official retail Apple Store) does not want to work
with you, be polite and ask if they can do the correct reset so you can actually own it...
Or replace it with a truly unused one that you can properly take ownership of.
• What to do before selling or giving away your Mac -- Apple Support
The items the seller would have to do to the computer you have, would be something
along the lines of what is said in this Apple support article; same if anyone sells or gives
away a Mac that previously has been personalized and is associated with an AppleID.
Even if you had the ID, the process to remove the original owner or registered ID from
the hardware and accounts associated with that person, is something you normally do
not need to do. Same if you had an iPod, iPad, iPhone, Watch, etc and it was used by
a retail customer then resold, traded, given away; or was a store demo sold at discount.
They cannot be used if associated with an Apple ID unless the original owner can reset
the product and even then, sometimes they won't work for a new owner.
With the other iDevices, the process is a bit different; for similar informative example,
I have included the associated article for those products here as well.
• What to do before selling or giving away your iPhone, iPad, or iPod touch -- Apple Support
So the seller should do the right thing and exchange this used unit that has been associated
with someone's personal AppleID accounts, and replace it with one that has never been used.
Or fix the problem so you can use it. Perhaps even at a discount since the unit is worthless if
you cannot be the person who bought it or set it up to their identity and with their Apple ID.
Good luck & happy computing! 🙂