I bought a set of three Magnalite anodized fry pans(used) and was told they needed to be cleaned out and re-seasoned. What would be the best way to remove the old seasoning?
If you mean that the anodization has chipped and you have bare metal showing through, the best way to remove the remaining anodization is by sandpaper and lots of time/ effort (aluminum oxide, a.k.a. corundum/ ruby/ sapphire, is one of the compounds used in sandpaper, so it's sort of like grinding a diamond with another diamond). You could also likely dissolve it with concentrated acids or alkali, but these may also destroy the pan in the process, and I do not know that I would want to do that to cookware.
The most challenging problem lies in how to put the anodization back on (very controlled re-oxidation of the surface), without a large vat of hazardous chemicals or investing more money than the pan is worth having it commercially re-anodized.