I never undercut and don't buy from people who undercut of like, nothing. If you want to undercut, you need to give at least 5-10% of undercut on the price, if it's something stupid like 1 gil, I skip those person so they don't sell fast. I sometime had people whispering me to ask why I was not buying from them and taking from people who were selling for more, and everytime it happens I'm a bit happy to see someone got pissed because they were thinking to sell fast and are not in the end. I always said, have guts to undercut, or just put the same price than the cheapest one.