To make money. It's just like real life shopping. You buy the cheaper thing.

Although there are stupid people who undercut way too much.

If something sells for 200,000 and they undercut to 10,000. Well guess what you just lined my pockets. Ill buy theirs for 10,000 and re list it at the current asking price of 200,000.

I only undercut by 1,000 to 5,000.

Selling is so stupid with the vast number of people listing things for only 1 gil. Like how pointless is that XD