On pretty much ANY public sales forum (like C/L), the deals do not last long. The best deals around here last 15 minutes, maybe 30 minutes max, if properly marketed (pics, and some decent but not great details). In any decent sized areas, there are always people on the lookout for deals, these folks are educated enough to pounce on the deals, any time of day, any day of the week. In my area, sometimes I am that guy. Often, I get beat out as well. I call these folks "scoopers".
So how do you "beat" the scoopers at their game? Get educated UP FRONT, so you recognize a bike from even a crappy picture, or sketchy details, and you become a scooper. The sketchier the details, the better chance you have of scoring a deal. A lot of scoopers are lazy, no pics and minimal/no details, they look elsewhere. I love ads with no size, no pics, no description, etc. Or if they do have pics, crappy pics, flat tires, incomplete bikes (missing saddle, etc.)
That's how the deals are picked up.
And often, the best deals are on bikes that need work, but only if you have the tools/time/aptitude/access to parts to do the work yourself. Otherwise, such great deals become lousy deals.
Of course, your other choice is word of mouth. That's really the best route if you are patient. And remember, a great deal on a bike that is not your size, can still be a great deal, if you resell it and use the proceeds to pay full market for a bike that is your size. All of my keeper bike purchases have been funded by bikes that did not fit.