I always make sure the price is agreed on before anything moves forward. Like if they are about to leave the house to come meet I'll say "so just to be sure it's $200 right?"

I have zero time & respect for people who think they can pull a fast one at the last second. This hasn't happened too much to me personally. But recently a guy drove across the city to get pick up a tablet from me. The tablet was very competitively priced and I was getting a lot of replies for it. So the guy meets me at a Starbucks near my place (always meet publicly for higher priced items). And once he agrees he likes the tablet, he low balls me by $40 - of course this is after he agreed and confirmed to my $300 firm price.

I quietly and firmly said "sorry you wasted your time, we had an agreement", and walked out of the starbucks. He quicklu followed me out and said something along the lines of "hey buddy, no no, I can get the rest just come back I was kidding" And then more angrily "hey man I drove 40 minutes to get here that's not cool". uh huh. I didn't care, and went home (2 minute drive for me) and let the next guy on the list know it was still available. He came and picked it up that night.