I built a small aluminum hardtail 29er if you're interested. It's basically new and is my "budget" build project. The frame is a Gravity small aluminum frame. The drive is a 1x10 with MicroShift AdventX 10-speed shifter and derailleur. Brakes are Tektro Draco hydraulics. The fork is Bucklos with air spring and 120mm travel. Tires are Vittoria Barzo 29x2.25 F/R with WTB wheels. Only had 1 real ride on it and it was a 14-miler at 6-Mile and it held up well with a 190-pound rider (me). I originally built if or my son but he's not interested in mountain biking, only likes to ride him BMX bike. $500 and it's yours.
I have a Schwinn Sidewinder that I bought for the wifey who only rode it (on the street only, never saw dirt at all) for a week or two around the community before deciding biking is not for her. Asking $175.