How common are mid-stage small software companies with co-founders looking to sell some shares? I'm not talking about anything near large enough to warrant registration. It doesn't seem like a lot written about this topic, because the focus is always on acquisition.

Obviously there's strong pressure from many directions to keep the cap table simple, keep the shares closely held, etc. But after many years of bootstrapping it can't be that uncommon to want to capture some of that value.