The best way to deal with the eBay comparison type people "without sounding like a total dickhead" is to educate them properly.
Bostonpinball.biz or pinpedia.com is PROOF what they actually sell for on eBay.
I cannot tell you guys how many times I've emailed someone with something like:
Hello! I am inquiring about your CL ad. I'm always willing to pay fair market value. So what is fair market value of your machine you might ask? (then I provide a DIRECT link to the game via pinpedia)
Closing with:
This is simply an average of what this title sells for. Depending upon the over all condition it may be worth a little more or a little less. I don't believe in tire kicking or low balling someone to death. I would appreciate the opportunity to come see the pin.
Please feel free to give me a call to setup a viewing time.
Now clearly if you want to low ball the hell out of them then this approach won't work for you. Less is more.
You can actually smack someone down and have them thank you for it. You'll be surprised how many people will LISTEN to you because you've been forthright with information that they were attempting to replicate (and failed to do so)
Telling a seller they need to put down the crack pipe isn't going to get you anywhere.