Once you have your keywords down, you can narrow or expand your searches as needed. I prefer to search by listed size (e.g. "54", "21", "55", etc.). Metric sizing is your friend because it tells you that the bike's owner knows their bike, and is unlikely to return a number of 'speeds', like 21 does. The art of it comes into play when you have an ignorant seller that doesn't list or describe their bike accurately. You can take advantage of that ignorance if you can find them. They'll list their bikes as "large" or "medium", and all bikes that aren't carbon are "alloy" (technically true, though I prefer to designate steel as "steel"). Wheels are always 26", which is occasionally used as the bike's size. Real deals can be found that way!

Final tip: If you can cruise yard/garage/storage sales in the ritzier parts of town, better equipment can be bought for less. The owners are usually just interested in getting rid of it, not trying to make a buck.