Raw materials and armours, or anything that's relatively price stable but has a good demand.
It takes some money to get started, but flipping mass amounts at once works great.
example buy a rocktail 5 percent over ge price, then sell it for 5 percent under. Check the GE history, buy rocktails at what you sold it for (the minimum price, offer a few extra gp per fish to save time) and sell them a few gp under what you bought it for (the higher price)
If you offer a little more when buying / sell a few gp cheaper when selling you can save a lot of time trading for little profit lost. The limit every 4hours is 20k and if the margin is about 20gp expect a ~400k profit.
Cheers. This works for any item that has a good demand

I do this with a couple items at once.
Won't be enough to flip many things at once but you can still make good money off one or two things at a time.