I don't want to sell some one a knight figure and have them tell me that the hair is fake.
When you use the term "fake," that immediately makes me think of parts that are not original Playmobil.
People (even children) often swap parts on figures. Hair is especially easy to remove and people sometimes forget which hair goes on which figure afterwards. Changing parts doesn't make the figures fakes. It just means that they have been customised.
Most people buying figures on eBay will not mind if you have changed some parts as long as they can see clearly what they are buying. The only problem might arise when someone sells a customised figure and tries to claim that it is a rarity or an exclusive.