I would hardly call myself a flipper, because I don't come across enough deals to flip bikes all that often. I will flip a bike here or there though and I mostly stick to low and mid-range bikes because that's all I seem to find in my area.

The low-end bikes don't have much of a margin, but sometimes all I have to do is clean it, maybe steel wool some rust, replace a cable or 2 and put some nice large pictures and a good description on CL and I can double my purchase price. All for maybe an hours worth of work. I try to snag dirt cheap low-end bikes regardless of condition to use for parts too which helps.

I won't do a complete overhaul on a low-end bike that isn't worth it, but I do find it enjoyable and worth my time to get it in rideable condition even if I only make $20-$50.