I was recently in the market for a 2nd hand road bike. I tried the trading post as well as Ebay.
My target was a bike with STi shifters, hopefully Shimano 105 or above throughout (or Campagnolo equiv), had to have drop bars & versatile gearing. Price was pegged at $1200 max.
Result = a Crosslite road/cyclocross alloy frame/forks with an Ultegra triple, 2003 model.
The problem I found with hunting 2nd hand roadbikes is that massive mileage is relatively easy to hide on a roadbike when well cleaned &/or chain/cogs/brake pads are changed. Gouged rims on the braking surfaces, play/noise in the bottom-bracket (bearings where the crank spins in the frame) & sloppy shifters can reveal a bike having a life well lived & $$$ fixes soon to come.
My advice to you is go to a bike shop & have a short ride on a brand new bike. Then go looking at a 2nd handy, with how the brand new bike rode as a guide. You want the closest “new bike” experience on the 2nd hand bike.
Keep in mind that bicycle parts are pretty expensive [for what you get].
Always test ride the bike before you consider it. Some people have a very different idea to “good working condition” than you might have.