1. Don't just stand there talking to the artist at their table! They can't run away and you block their display from other con-goers who could potentially be customers if you weren't blocking their way.
I had this happen twice, both from older men. One talked to me for thirty minutes and blocked the way when a visitor stood next to the table for ten minutes, waiting for him to leave, before finally giving up. I just lost a sale, and possibly others, but I visibly lost THAT sale. The second time, the guy talked with me for an HOUR... He showed me vacation trips on his iPad! If you want to talk to an artist, talk to them about them or their work, not about your trip to Paris in the spring.
2. If you do take up a portion of their time, for the love of all things, buy something!
One man I talked to bought a $12 book, but the other man who took up my time didn't buy anything. Something similar happened with another client, who pulled out a $100 bill when I didn't quite have enough change for him yet ($100 for a $12 book? Carry smaller bills!) and it was rather frustrating.
3. Don't hassle other con-goers to buy the artists stuff. You are not their private promoter!
I had a man grab a girl walking by, shove one of my books in front of her, and he forced her to read it, talking about how amazing it was and how funny the story was (he hadn't even read it yet). She looked scared and confused, and wouldn't you know, when he left the girl didn't buy anything. I apologized to her but she was clearly shaken.
For artists: How do you deal with customers with bad table manners, so to speak? Do you ask them to move along, or do you just chalk it up to part of the con experience? I absolutely could not get the man showing me his vacation photos to leave through hints and body language, but he bought something and I didn't want him to feel like now that his money was mine he was of no use to me...