Here's a good suggestion. If you are buying something and you are among the group of people who hate undercutters just buy the higher priced item that is more in line with what you believe to be the fair market value. However if you are buying up mats/gear and you purchase the bargain priced items you don't have a leg to stand on.

The way undercutting thrives is that people will buy at the lowest available price, then again why wouldn't they?

That being said I only undercut by maybe a few hundred gil unless I see obvious extortion pricing going on.