In my opinion, and where I live, $100 for a Singer 99 is a little too much. They are definitely worth having as a second machine, but typically these sell for about $35 at garage sales in the metropolitan area. For $100 I would expect a machine with near-perfect decals, good wiring, and a cabinet or wooden carryall in near-perfect condition.
The featherweights can usually be quickly identified because they have a little hinged platform to the left of the needle; this platform can be folded up to make storage more compact. Plus, they are light and easily lifted with one hand. (Other Singers are much heavier.)