Add a listing fee. Make it 1%. Remove retainer fee. A 1M gil item would then cost 10k to list and another 10k to relist. That would make people think twice about undercutting.

Alternative or supplementary fix? Add a place to advertise what you are selling. Similar to how some people are using PF for it (ugh) but make it separate from PF. And I'm sure people would prefer NOT to see it spammed in shout. I'm guilty of using PF to sell or buy stuff. I like the concept of using it, but just wish it wasn't in the same PF that people use to make groups to actually do stuff.