Buying used,unless off a reputable cleaner for example,it's best to assume the worst. Chances are the vacs and or pump won't last long. It may have been sat not used with damp air or with the lid on for example which will shorten vac life a lot. Always worth seeing if there is any dust/dirt around the vacuum exhaust,that will give an indication of whether it's been used with water through the vacs . Aside from the vacuums and pump it's only damage to the tanks ,seals or gaskets and the wiring and internal hoses .check for leaks ,rust on connections and also wires - some crumble away over time I have seen