right size, right size, right size. Don't get a bike that doesn't fit.

You're a light person so look for a light bike. Low standover height will put you into looking at the smallest mtn. bike and mixte frames in the used market. Get a tape measure and KNOW what your bb to seat height should be. Don't go by used bike sellers statements on frame size. I got a GT 26" wheeled mtn. bike for my daughter that fit and still fits her 8yrs later, I think it was around 13"-14"inch seat tube. I simply added a longer stem as she got older. With 1.5" tires it was actually quite light. Most of the Craigslist bikes shown above are too big.
If you don't know anything and you're wiling to go to $400 go to the bike shop and be straight up, "what do you have under $400". While you'll get something cheaper used you'll pay for it later if it doesn't fit or you have to bring it into the shop and pay $150 for minor repairs like a rear wheel replacement or drive train replacement. In other words if you're willing to spend $250 for used it better be nearly perfect and you know it, if you don't know it you should be looking at $100bikes and be willing to spend $150 for an education in possible repairs. With a $400 new bike you're getting some worthwhile free maintenance and a warranty.

If I was in your shoes I'd look around for the cheapest step through or XS mtn bike for under $100, put a decent puncture resistant 1.5" tire on the rear, cheapest 1.5" the front and spray the thing flat black with a $4 can of paint