Do u have 600+ auctions on same kind of material like always only ores or only gems etc.
And also, do he that undercut u always have the same kind of materials?
Does he buy or only undercut?
I would sugest u to try to trick him...
Put an item he have very very cheap and see if he undercuts or buy it out.
If he buys it he's using a smart bot or he is doing it by hand.
If he undercut with all his materials u buy everything from him and get rid of him that way, u also get profit that way xD
I would try that if i where u cuz it has succed many times for me and if its only 1-5 gems or whatever its not a big lost