Hello, Dave and Ctx are right, don't use any detergent , just use distilled water and very mild dish soap, and gently wash it off, make sure it is distilled water as the chemicals in other waters can affect the coin chemistry, then use only cotton cloth to wipe it dry, don't use other material. if it is too dirty soak it in olive oil for a couple of day and then do the cleaning process above.
if it is too valuable and you want it cleaned by a prof, search for them in your city, otherwise leave them as they are,
if they have green slime on them by being infected while sitting in slimy waters etc, soak them in a glass bowl with acetone for a minute or two then wash them with distilled water and be careful not to rub or anything of the like just pick them up with tweezers from the dish and put them in a distilled soapy bowl, dry with cotton cloth, that is it, the buyer will have to do the rest if he wants to after buying them but then it is out of your hands , hope this helps ,
cheers flatlander, anyways, here is the website where I got all this info, you can read it if you want, I use it all the time , and I use this method to clean my coins too
https://www.buysilver.org/how-to-clean-coins/