I ran into a remote AH competitor last night. He will constantly undercut if he is online. but I noticed that he was also doing it while he was offline. This means that he is using the mobile AH. As GI Joe say, "Knowing is Half the Battle". What do I know? The remote AH only allows 200 transactions per day.
I would post and within 30 seconds, I was undercut.
To combat this, I changed the number of gems I post of each kind to a single gem. I would then post. Wait for him to undercut. Post another round of single gems. This causes him to lose a bunch of AH fees as he has to cancel them again. It also depletes is remote transactions for the day.
It forced them to get back online. Causing severe inconvenience is a legitimate way of eliminating competition.
No go forth and make gold.