https://support.gunbroker.com/hc/en-us/articles/224766967-Buyer-s-Protection-Program

The item is most certainly “materially different than the description” so I’d think that the buyer has some recourse.

I realize that the buyer is not too knowledgeable about Mauser, but the photos clearly show that this is not a magnum action and also show the modifications that it had suffered.
Although the action length isn’t ll that easy to discern, one can see that it does not have a straddle floorplate, which all magnums have.
Another help in determining the difference is where the front action screw is in relation to the receiver ring.
The screw threads into the lug on the bottom of the receiver ring. On standard actions, this lug is just forward of the vertical midline of the receiver ring. On a magnum length action, the lug is behind the midline of the receiver ring.
It is easier to tell when the rifle has a crossbolt in the stock because the crossbolt is directly behind the lug (where it does its job dissipating recoil). On a standard action, the crossbolt is close to centered under the receiver ring. On a magnum, the crossbolt will be very close to centered under the rear edge of the receiver ring just in front of the magazine.

I’m pretty sure we discussed all of this at length here on Nitro Express some years ago.

All of that said, this seller is epithet a crook, an idiot, or that special combination of both.

Here’s the ad:

https://www.gunbroker.com/item/1034173843