I've bought 2 bikes for my wife off of eBay. Not to say that is what you'll want to do. But the first thing, after fit, is to find a bike that has not had much use. They are out there. People buy bikes and they spend most of their life sitting in a garage with little use.

The pictures of my wife's Trek 8000 mountain bike did not show any scratches and it looked like black chainrings had no signs of use. This was 5 years ago and I paid $460 shipped. It really needed nothing but personal customizing. It is good to stick with a "brand" name such as Trek, Giant, Specialzed, etc... with maybe the exception of a good house brand.

Her other bike is a Performance brand, Scattante, that was purchased by a husband and never ridden. That cost the same. I'm only bringing up the Performance house brand because it is a good bike with low resale value so used should be cheaper. I don't know your budget. It is not a Walmart level bike.

John