As posted buying the correct size is job one! The best bike does you little good if it doesn't fit.

CL is an excellent resource for experienced riders. These are people who know bikes and bike components. If that's not you it would be easy to get snookered.

Since that is a problem what to do? How about a new bike at a steep discount? many bike shops have last year's models still available. Most shops will offer deep discounts on these bikes. There are shops that have models that are several model years old. Mostly, these bikes are in the lower and upper extremes of fit range. This weekend i was in a shop that had discounted models ranging from 52 to 58cm( 58cm is in the sweet spot for fit). These were road bikes. One example was a Trek road bike ( can't remember the model) normal retail sale price was $969 marked down to $599. That's a deal!

Other possibilities - Goodwill.

Lastly, if you are near a resort area many bike rental companies sell the fleet between seasons.