You'll need to observe the market a bit, but rule of the thumb is: when new tiers are out, new craftable epics and new epic loots will have a decent chance of making cash as long as you don't over-invest; otherwise, it's the following for your day-to-day profiteering:
1) Consumables (food & booster pots primarily)
2) Crafting materials (usually low profit margin, but you can usually grind/craft something out of those, or convert to consumables. I made thousands in WotLK just buying and cooking fish, buy a stack for 25-50g, sell for 250g)
3) Unique gear (may take a while to get a buyer, but if you see something that can't be crafted anymore, for example...)
Otherwise, hard to say. Get some addons like Auctioneer, keep scanning daily, build up a solid base for your server average prices, start sniping.
Fake edit: Or addons that
Revenant mentioned.
Just watch out with the epic purpls - they have a limited time of opportunity for making a profit with, as the post-patch days move along, the prices on those will usually collapse, so never, ever bet on the purpls market unless you know what you're doing.