I would suggest finding old pickups in music stores and taking them apart and examine the components used in various models. You will see coils that look the same but some use a finer gauge of wire than another and the magnets may look alike too, but they can be a different grade and strength. Take a good look at the materials used in the injection molding of various bobbins and soon you will be able to see the types of quality the builder puts into their pickups. This will really help you in determining the quality and reliability of a pickup.

Take a good look at the winding of the coil and wear on the bobbin. Oxidation or rusting of parts is pretty common especially with parts made of a plated ferrous material. The ferrous parts are needed to conduct the magnetic field from the magnet to the strings and may rust due to humidity and sweat.