Separating the gems from the trash
There's an awful lot of overpriced crap on my local CL. The vast majority of the used bikes are junky mass-merchandiser bikes. Even though the names may be familiar, the bikes sold by box stores are mediocre new so they will be mediocre or worse used. You can screen out many of them by listing a low and high limit in the search.
Most of the worthwhile road bikes on CL are high end and the asking prices are far above what you have budgeted. If you know the make, model, and year of the bike, you can go to www.bikepedia.com and look up the list of components and original MSRP. You get no warranty with a used bike so the seller should give a good discount (many don't!). The used bike price should be significantly off the MSRP and be greater as the bike gets older. Doing a search will give you an idea if the price is reasonable. Often you can easily find online reviews by simply searching on the brand and model. You might do better with a decent quality older bike.
The best used bikes I have found are located in thrift stores and garage sales. I know what is good and what is not, so that makes choosing easier. I qualify for the geezer discount that a lot of thrift stores give so that makes it even sweeter