I wouldn't be surprised if, sometimes, there are limits placed on this sort of thing, especially if the author is well known and they expect tons of people. At a signing at a regular book store, you ask the author to sign the book, and then just go to the cash register and pay for it. At the RWA conference, there's a whole bunch of people processing cash and credit card payments. There's even people directing traffic to the next open check out. I've never dealt with anybody who's been over zealous, one way or the other.
Most people are nice, friendly.
If they tell me they like the book, I thank them. If they tell me they didn't care for it, I'd probably say I was sorry to hear the book didn't work for them, and better luck with the next one. I have had the occasional crazy person approach me at signings, though. I was SO glad I was at a busy Waldens, and sitting with another author. There are no responses to this message.