Without the grand exchange, flipping and merching is insanely beneficial.
Even if you're not experienced in it, it's very easy once you get an idea of the prices.
I once bought a rune kite for 130k and sold it 5 minutes later for 170k.
(That was pretty lucky, but I can usually make 10-30k profit between flips)
A good piece of advice is to broaden your scope between in-game, IRC and Zybez market.
If someone is selling something for 100k in #2007trade which people are buying at 110k in world 2, you have opportunity.
Always trade for cash, and avoid taking other items in return unless you are 100% sure you're not being ripped off.
Make note of what items sell best in what worlds. For example, Dragon Scimitars seem to sell best in world 305.