Apparently most of us can't, or it wouldn't be worth using them. ;)
Really you could consider it a subset of truth in advertising law though. How can it be a 50% discount if it's never been sold at full price for a significant amount of time?
Basically what happened is that I did sell it for $8, then I did a 50% sale and saw an increase in sales. I did the sale on and off a few times then just got lazy and didn't get around to removing the sale flag from the website for a while and eventually I just decided that most people only buy it once, so from their perspective it's on sale now. It also saved the effort of remembering to switch the pricing back and forth every few weeks.
Really you could consider it a subset of truth in advertising law though. How can it be a 50% discount if it's never been sold at full price for a significant amount of time?