That looks like one of the older Max 120mm or 130mm models, can't see fomr the pic but does it have a big red dial on top with a kind of vertical crescent protrusion? or a small red knob with knurled or machined edges?
It's definitely an older model, which are desirable for some reasons, dampers are easily serviceable, removable clamps etc, less desirable in others, heavier, harder to convert to normal steerer etc.
Used examples of the above go for anywhere from £100 - £200 depending on condition, newer carbon ones are now going for £250 - £300, and the even newer PBR damper Alu ones can be had for < £300 now 2nd hand as well which is pushing the price of the older Alu ones down a bit at the moment.
Still, it's in great condition so that's a massive bonus as a lot of what puts people off buying them 2nd hand is the potential for having to get them serviced.