Just how rare is a VS Slalom? There is one for sale on ebay for $1199. I have never seen one before or even heard of it. Looks like the ultimate addition to a Nintendo row, just not for that price tag.
They turned up more frequently in the early 90s. It's not as rare as it might seem, but a lot of ops did trash the ski attachment. The trouble with them was that the attachment didn't do much to enhance the game, and the game isn't terribly interesting to begin with.
If it's like the NES version, it's probably pretty terrible. I don't get the point of the ski poles either as there isn't much to the game. The attachments are rare I'm guessing, but the cab itself is only worth $250-300 depending on condition.