OP..where are you in Wisconsin? Are you willing to travel and hour or two, or three, round trip to pick up a bike(greatly increases your chance at something nice)?
What size bike are you looking for..16 inch? 17 inch?
There's tons of mountain bikes out there used and some are quite nice. While prices have inflated a bit with covid, in Wisconsin I don't see inflation to a large degree(I scan CL for various types of bikes daily, and have for years). Folks can ask whatever they like, but selling prices often are lower. Two friends & myself have purchased used bikes in the last two weeks for fair or
very fair prices. In spite of the impacts of covid, demand is dropping as the temps drop, and prices drop with it.
A fully rigid bike might be best as used, lower priced hardtails often have iffy suspension forks when they were new, let alone well used. If you can find a garage queen then a hardtail might be good. It continues to amaze me how many like-new bikes are available. I bought a bike over the weekend for a friend (xmas present from her family) that has the original tires on it and given the look of a fine center-ridge on the rear tire, the bike can't have 100 miles on it & it's a 12 year old bike.
The Trek 900 (930, 950, 970, 990) series and Specialized Stumpjumper & Rockhopper are safe bets and higher-end bikes. The models lower than these (800 series or Hardrock) are less desirable(from a component & weight standpoint) and won't sell for much less..target higher-end models. Shimano Deore, LX, DX, XT components are good..and also make for a very easy/inexpensive drop bar conversion if that's of interest.
Took a quick look in this (our) region(sizes vary, but are pricing examples) is below. The Minneapolis area continues to be a used-bike heaven..so many bikes in this area it's amazing.
This one may well fit you..and looks like a fun ride if the shock is ok or could be updated:
https://minneapolis.craigslist.org/r...212406393.html
https://minneapolis.craigslist.org/r...217465489.html
https://minneapolis.craigslist.org/h...216890783.html
https://minneapolis.craigslist.org/h...209409684.html
https://minneapolis.craigslist.org/h...212711618.html
https://eauclaire.craigslist.org/bik...204130263.html
https://eauclaire.craigslist.org/bik...209724124.html
Very nice bike..can't imagine why it hasn't sold..been listed for a month
https://mankato.craigslist.org/bik/d...188896489.html
https://minneapolis.craigslist.org/h...196969422.html
https://milwaukee.craigslist.org/bik...189942275.html