I think it's silly to think that in today's advance of information and how hard it is to route a pin that an operator would not know the value of his machine.
I also think there is a lot of "old" experience is being told on this topic. As an operator I take exceptional care of my pins, all of them play flawless and all but one is beautiful inside and out (it plays super nice and is fully functional) also I am legit with a LLC, proper records keeping for taxes and not just putting pins up from my home.
A lot of the picture that has been painted in this topic is a dirty cheating operator that is evading taxes, doesn't take care of his equipment, who is very angry all the time, hordes equipment and sells to know one due to possible competition. This is not the case today.
I am happy to discuss selling a pin off route, not because there is something wrong with it, it's good to change the line up. I am happy to meet and discuss, break down the pin, help load, answer questions, but only to a serious buyer. I also encourage other people to become operators, I have helped one guy become an operator and helping another right now. I don't care about "competition". There are more places to put pins in today than operators can keep up with. I love to promote pinball and would love to see pins everywhere!
I will say this, don't go and offer up $1000 less than what they sell for, when a pin is sold another has to fill its place. It doesn't make since to take a loss like that. Also, don't offer up trades that are a lateral trade, it doesn't make since to move a good working and earning pin for one that is unknown, needs prepared for route and has a lot of leg work in moving stuff around. If it wasn't listed for sale, you have to offer up a reason to sell. A humble buyer will get a better deal from me than a know it all, aggressive one that is going to be a pain in the rear. I think most ops would sell if they weren't worried of having to deal with the pin again when it leaves.