My Wife began keeping detailed spreadsheets of all the bike flips I was doing. I am not so organized and did it for fun and to make a few bucks. After doing a few with her record keeping (money and time), I was always coming out ahead but some bikes (and I would take anything), just were not worth the time and effort, especially if tires and tubes were needed. Other bikes were amazing cash cows and those feel great to restore and sell to a new happy home. This summer, I had a few quick wins with yard sale bikes. Buy them for $20 and spend two hours cleaning, lubing and tuning and sell the very next day on Craigs for $100. I have found that anyone will pay $100 for an older but working bicycle that is not a box store brand. I could have held out for more money on some but the hunt and quick turnaround is really what I enjoy the most.

Overall, the exact record keeping and profit sheets that my Wife keeps takes some fun out of it. That's her thing though...