While it is very possible to clean your pennies without damaging them, it is hard to say the same for the market value of the pennies. When you clean via any method, you are clearing off small parts of the penny surface along with the corrosion, so a slight change in the appearance of the pennies is actually inevitable. You may not see these changes with your naked eyes if you clean carefully, but they are there. You just can't see them because they are microscopic—but your intended buyer will see them.  Collectors actively look for the tiniest flaws in pennies and they have all the necessary tools to find those flaws.  So our advice is that if you have found some valuable pennies (and have hopes of selling them for big money), it is actually safer to not clean them at all.  It might even be a good idea to stay away from even hiring a professional service to clean them for you.  Sell them the way they are (or “as is”) and you will surely gain more money from your pennies—well, assuming they are actually worth something, of course.