well in this case I was very lucky that the second bike needed very little (mostly some adjustments and truing the wheels). The first bike I did a ton of stuff including stripping off the house paint that covered up the frame, rust removal, replacing bearings, in addition to the typical repack and recable.

but yes the concept is buy things below market value, add value if possible, and then resell them at market value. It might mean buying something and fixing it up, or buying things that are presented poorly and then presenting them well. If you do right, you can sell the bike faster at a higher price than the first person would do it at a lower price.