Sometimes the market for an item is low enough you can buy it all out, post a few for a large markup and make unusually large profit margins.
It's all about supply and demand, if there's steady demand for something with both supply and price low, it might be a good time for a buyout.
This tends to work best on relatively cheap stuff, someone might pay 25 or even 50 million for an item that usually goes for 1 million (especially if he's just gotta have it RIGHT NOW) but it's highly unlikely anyone will pay 1 bil for something that usually goes 100 mil.
MHVO-Is usually go for around 1 million ISK each. I bought out 100 or so of them for a bit less than that. Then I posted 10 for 25 mil each. I didn't get undercut for about a day and 5 of them sold before I did get undercut.
I already made back the 100 million I paid for all of them plus 25 million in profit and I still have 95 of them left (5 more of which are up for around 10 million each now).
Not bad profit for buying a stack of things I keep in stock for my own use anyhow at a price slightly lower than usual. It's not the first time I've done that on MHVO-Is either. Last time I reposted them for 50 mil each and only sold 2 before getting undercut.