Thank you, this gave me an idea on how to go about my next sale.
I've also had sweet deals and I believe giving back is a good thing to do.

If I were to sell one of my machines at the price I paid for it, there would be tons of inquiries and most of them probably from professionals that skim the classifieds pages on an hourly basis.
Since I don't want the buyer to flip the machine for a quick buck, I will post a firm price that may be a tad lower than current sales prices of comparable machines.

So here's what I intend to do when a buyer shows up:

If he is nice, wants the pinball machine and doesn't talk it down to get a lower than the (firm) advertised price, we will close the deal. After he's paid up, I will give him a couple of hundreds back which will surely brighten his day since he got himself a sweet deal. I will also encourage him to do something good for someone else who doesn't expect it (pay it forward).

If he's being a jerk (e.g. takes out the cotton swabs and complains about anything to get me to lower the asking price), he may still buy the machine from me, but it's going to be full asking price without any kickback. In that case, I will donate to charity what I would have returned to him.