its a buckling spring,take a look at one of the pics where keycap is removed and you will see the spring sticking out, By the way this is a good seller,got from him BOTH BRAND NEW Dell AT101W and IBM Model M just without the original factory boxes, The IBM had factory tape around the keyboard and the cable was still wraped in the plastic baggy, while the dell was in plastic bag with some documentations, no boxes but both brand new at $130 price including shipping.
EDIT: Offcourse I had to discuss with the seller and come to an agreement for the price on both boards.