I always include the manual if it came with it when I bought it. One time only when I did a nice restore on a WCS94, I bought a NOS manual and included it in the sale price. If it is an older game, I will download the main parts, print them out, and include it in the sale price. I do not always include a bunch of extras like slingshots, ramps, etc. Here is a funny example of why not.
I once sold a very nice MSF to someone many states away so shipping was involved. The game was REALLY nice. For some reason the guy saw it necessary to talk me down $50. I had a NOS servo board for the Frankenstein head which I was going to throw in the deal but since the guy talked me down I kept the servo board. So the game gets sold/shipped/arrives and everyone is happy. I then put the servo board on Ebay and it sold for about $75. Guess who the buyer was? Yep, same guy that bought the pin from me!