First thing of course is the correct sizing from a pitch standpoint. Diameters can be reduced but rairly added.
Next, why are they for sale? Have they ever been repaired or tweaked? How did they perform on your boat pre repair/adjustment and how did they perform after? Inspect leading and trailing edges for cracks or porosity. Look for obvious damage. If large HP applications are involved, magna-fluxing is a good idea. Ask how old they are and if they are the original owner.