People aren't irrational only when buying on eBay. We are pretty far away from being rational even when thinking about prices.
A recent paper in the Journal of Consumer Research indicates that we tend to act as the lower digits were farther apart than higher ones.
"Students who saw ads showing a $233 skate marked down to $222 thought
they were getting a larger discount than did students who saw a $199
skate marked down to $188, even though the opposite was true. The first
group of students also rated themselves about 20 percent more likely to
buy the skates than did the others."
We aren't as clever as we tend to believe. Irrationality is very important when observing and analyzing human behavior. It also makes advertising kind of difficult as it is hard to predict, control and influence irrational behavior.
Via NYTimes
