Yard sale ethics are always an interesting topic.
When buying games, I try to judge it on the day I am having, who the seller is, and on how my most recent encounter with competition has been. For example, if I having spent much time going around or haven't driven around much or eaten, I think I am more likely to pay the asking price or talk down less. If the seller is a kid versus an adult, or if they don't begin by stating something along the lines they want this gone, I think I try to pay more for games. I also try to pay more than competition in case someone has more games and calls / texts later, that way the competition doesn't try to persuade them to be called instead.
I am also more likely to buy a lot of common games if I get a harder to find one in the process. For example, this morning someone had a Raiden Project PS1 complete for $2. Instead of buying just that one, I bought 6 for $10, including games I would not have bought otherwise as $10 was more than fair for Raiden Project and Resident Evil 2. So I ended up with some filler games I do not believe I have that I normally wouldn't have paid more that a buck for - Pitfall 3D, Activision Classics, Wipeout XL and a disc only Wii Sports Resort (the 'free game').
I did start recently to pay more than asking price if the deal is stellar (as long as the seller isn't rude) and I hope to continue this trend as I do not want to feel bad about something.
For items in less demand, such as comics or magic and pokemon cards, I tend to negotiate a lot more as those items are more condition sensitive and people usually have a better idea of what the actual value is.
That being said, this is just my opinion and I do not blame anyone for getting a cheap price as long as they aren't being an obvious jerk.
I do not believe its right to snake a deal (being the first one there in cutting in line, offering more than someone is trying to negotiate for), 'spreading out' over items, trying to resell what you buy on the spot, using one of the cliche'd lines that isn't true that we have all heard, and many other jerk like things that I am sure we've all seen.
**Edit**'
I didn't realize Raiden Project was a $50+ game. I thought it was half that max.