Regarding the conditions of sale, I have no idea what rules govern facebook sales, I do not even have a Facebook account.
BUT on the likes of ebay items are sold in various condition categories, e.g. new. used, for parts etc. and definitions of those categories are available.
So, if this drone had been sold via ebay and in the used category it would be expected to work like a new drone and the condition category over rides any text within the advert. So, stricly speaking, if this were sold via ebay in the used category it would be returnable as, I think, "not as described" DESPITE the seller listing the tilt issue in the text of the advert. Morally I suppose you do not have any grounds for complaint.
I have, on at least one occasion, verified with ebay whether or not they would stand by the category rather than the text.
The above said, this may be a simple fix and if you use the search function (see attachment) you may find repair info. One thing I would suggest 'checking on' is whether the Mavic Mini knows the identity of the camera and or gimbal attached to it and whether a replacement gimbal/camera will create problems and, if so, if it is possible to overcome that.
The tilt you show is "roll" but the camera/gimbal has another two axis of rotation.
One, pitch, that allows the camera to point up or down and that is controlled via the dial on the front LHS of the controller, does it respond to that dial? If it does and roll is not a "must have" for you I suppose you could find some way of jamming the gimbal in the zero roll position and thus sort of have an permanent FPV view.
The third axis is yaw (turn) but I find that hard to see/check