The OP is not talking BS here... we have the exact same "problem" on my server. There is a goldseller-char that is online 20h a day and have 1 gem of every sort out on the ah, whenever it sells, the person put one new out. If someone undercut, he immediately removes it and puts in back for 1s less that last bid. This dude (well, must be more than one as it's online so much) have been doing this for weeks now and I'm pretty damn sure this is the Goldselling-idiots new strategy to earn money, stop people from making money (so they have to buy from the goldspammers) and basically ruin the server economy completely.
This is not the OP being paranoid or a jackass that cannot take some normal AH-competition.... This is the goldsellers new and incoming tactics for destroying the economy. Probably a pack of lowlow-paid idiots sitting in a cellar, taking turns to just scan AH, run between AH and bank and that's all they do!
You simply CANT SELL SHIT with these guys doing like this and I'm pretty sure the rest of the servers will get this sooner or later.
I've contacted GM's numerous times but they don't do shit coz it's technically not illegal to undercut 24/7, but I trying to get them understand that this is serveral idiots working on the same account and get them banned for that.... no luck so far... Blizzard doesnt care! Gimme a new game!