Some potentional buyers actually prefer the mufflers to be covered in baked on goo, then they know castor oil have been used in the fuel and thus the engine is likely to be in good shape. But I guess most people prefer a engine they want to buy to be looking like new on the outside. It shouldn`t take too long to just polish the mufflers clean and shiny. Use a power tool (drill, dremell etc) with polish bits and some compound. the crock pot method is effective but will take hours and is potentially harmful (fumes) and dangerous for the surroundings