First mountain bike for my 14-year-old

Hey everyone!

My 14-year old enjoys riding their hybrid bike around our suburban area. They’ve expressed the desire to start going mountain biking with me and I am really struggling with how to find them a good first bike. They’re not particularly good at bike handling, so I’ve been looking for something with a trail geometry and fork travel.

I’m wondering if (1) there are any hardtails worth looking at and (2) where on earth you can find a used bike at a reasonable quality and price point? My FLBS can cut me a deal on a Salsa Timberjack with a decent build, but it’s still over $1k US; online used bike shopping seems like a total mess (TPC: too expensive; Facebook/Craigslist: bad bikes, mostly stolen). Any suggestions?

Thanks!!