This is a question I frequently get when I buy pinball machines. Typically, the games have been sitting for years and are covered in dust. The playfields are dirty and there is a black firm on the playfield. This black film is flipper coil dust or carbon. The problem with this dust is that when the balls roll across the playfield, the dust gets ground into the playfield and causes scratches and damage. This eventually leads to the clear coat wearing away, then the paint wearing away.
How do you prevent this? Wipe with windex? NO. The proper way is to take the entire playfield apart. Meaning, take all the components, ramps, plastics - basically any piece on the playing surface off. These pieces are all then cleaned. The playfield is cleaned with a special cleaning agent safe for pinball machines. Then the playfield is waxed, the pieces put back on and bingo - you have a clean game. Now, how long does this take? Anywhere from 2 hours to 10 hours depending on the complexity of the machine.
So - when a buyer says to a seller..."This game needs a tear down and cleaning" - he means approximately 2-10 hours worth of work. Its simply not just windex and paper towels. Amusements retailers generally charge $40 - $75 per hour for labor and it can cost anywhere from $100 - $1000 to properly clean a machine. Most hobbyists, and buyers here on pinballvalue, do not attempt to knock this amount off the price. However, consideration should be made by the seller as to how dirty the game is and how much work is needed.
My rule of thumb - if a game is dirty, knock off a couple hundred for the buyer.