It depends on the person and the deal.
If it's a fair price I normally say something like 'I'm not going to talk you down, but I need to know everything about this bike'. I found that the owner is more likely to give me a fair representation of the bike if he knows I'm not going to low-ball him. If someone is asking $1000 for example I'll go there with $1500 or so and ask about any extra parts he has, express my desire to own the bike but say I need to test ride and really check everything out good. I've spent hours checking out a truck or car with tools, compression gauge, vaccuum gauge, you name it. I generally ride my FJR1300 to look at any bikes to show the owner that I do ride and am perfectly capable of test riding their bike without wrecking it.
If it's overpriced, or even worth the price but out of my range, I'll make my offer polietly. Something like 'I hate to say this to you, but I can give you $500. You might be able to get more elsewhere, but that's as high as I can go.' If they don't want to take it just thank them for showing their bike to me, leave a number, and walk away.
Over the spring I sold a truck to a kid for $10 because I got sick of lowball offers. It was a junk Ford Ranger that needed some work, I was asking $500. He showed up in a perfectly restored 86 Ranger, said he loved those trucks, and pulled out $500. We talked for an hour or so and I found out that he had plans for a frame off restoration on my truck. He had stopped earlier and looked up the VIN, found out what I had. My truck was built during the first month that ford starting building Rangers. The motor had blown in 1996, and the truck was parked since then. Perfect body and interior, no rust. I said heck with the money and let him have it for $10 just to see it go to a good home. It's partway done now, I'm just happy to see the truck get the attention it needed. He stops by every now and then, I sandblasted some parts for him and went to his house a few times to check out the truck and help.
