Originally Posted by
veganbikes
If you know you way around a bike at least enough to know if parts are damaged and worn and somewhat levels of quality you might find a decent used bike. I would avoid full suspension at that ultra low price and focus on a hardtail and ask loads of questions on the suspension fork and their care of it or prepare to spend some good money on either a service or rebuild or buy a nice new RS Judy Gold or Reba RL if you want to get spendier or a Recon Silver TK or Manitou Markhor if you aren't going to ride as often.
If I was going to buy a bike I would buy something new hardtail and look to spend around $1200+ but really the key is a good Air Shock from Fox, Rockshox, Manitou, Marzocchi (which is now owned by Fox), DVO, MRP...quality stuff and I want hydraulic disc brakes from Shimano, TRP, Magura or if I have to SRAM (though their new brakes are actually pretty decent but mostly DOT fluid still and that is a no go for me personally) I would want Shimano Deore 10 speed or SRAM GX at the absolute bottom end anything lower and I would pass. Depending on quality of other components I might also consider Microshift's Advent X groupset and would also consider say the TRP 12 Speed or the Ingrid stuff (especially Ingrid only becuase they do a 90s style Rasta colorway on their derailleur).
If you want full suspension be prepared to spend money on it a lot of the same above applies but price will double at least.
Also keep in mind used vs new with the new bike you can get warranties so say for Specialized you get a lifetime on their frames and all the individual component manufacturers have their own warranties so if something does happen you have support. You might also get a free tune up or two from your local shop and you know the condition of the bike and what has happened to it from nearer to the beginning vs used you may not know any history. Plus a new bike from a shop is not going to be stolen which can happen with used bikes. However if you do know what you are doing you can find some neat old mountain bikes that are still perfectly capable old XT is generally bombproof.
Your suggestions aren't necessarily wrong, but OP seems to have a budget of $400. Recommending spending $1200+ on a new bike is a whole different world, and probably also a big step too far for someone just wanting to give MTB a try. That said, finding a used bike of decent quality, suitable for real trail riding, for $400 is not an easy task.
My 3 latest bike purchases have all been used bikes, and have worked out really well. However, I knew what I was buying, and I wasn't scraping the bottom of the low-budget barrel.