On a high pop server your profits come from high volume sales with smaller margins. You let your buying and selling operations handle calculating the profit margins and keeping you from selling under your buy prices, but there's no way around it. You need to baby sit your auctions, sell in neat stacks and be ready to respond to undercutting. I have days where I'll check in every hour to process undercuts and unsurprisingly, those are the days I sell the most. Other days I can't be bothered and whatever sells sells.
Where I would be looking if I were you would be: Starlight Rose
, Slice of Bacon
- scanning for <70% auctions.
You also need to be running sniper and have a good idea of what the price ranges are for items you're interested in. Apart from that - as I said earlier, on high pop servers - it's all about volume.
Also take a look at the undermine journal...grab a list of mats you're interested in tracking and check their volume and pricing histories. That will give you a GREAT indicator of which mats on your server move, and in what volumes, on what days, and at what prices they move.