2nd hand tenkeyless is kind of hard... because the whole tenkeyless phenomenon is fairly recent, so there aren't a whole lot of "vintage' boards w/o numpads (unless you count the Model M SSK, which you likely won't find for <$100 anyway.)
I guess the real question is, what kind of keyswitches are you looking for? Even only considering Cherry switches, there's a lot of different flavors out there. Knowing what you're going to use the board for and what your typing style is will help making better recommendations. e.g. I like clears but there's tons of people out there that think they're too heavy, so personal preference factors into choice a lot, and if you are playing lots of FPS games and not typing a lot you may want to go straight to blacks, reds, or browns.