This is the ultimate way to get lots of money. I can't help you get the first amount of money needed.Start of with anywhere between 25k - 50k.Then go to karamja and there should be lots of people fishing lobsters. If there isn't switch worlds. then buy all of their lobsters. because most of them are only training up they will sell very very cheap (100gp -200gp). Do this lots and lots of times until you have about 100 - 200 lobbies (note: do not try this if you are afraid it won't work, this will use most of your money). Then go to a very busy world near the wilderness.
Good places are edgeville and varrok banks. Sell your lobsters for 250gp - 300gp, and that is one hell of a profit!

But wait. I'ts not over yet. You can still get alot more money. Do the first trick alot of times. By now you should have at least 100k. This is where the real merchanting skill takes shape.

Buy a bulk of lobsters (and i mean bulk!) for about 200 each. This is alot easier now because of the new grand exchange in varrok.

Then sell for alot more. not hard is it? This should make you happy! Happy merchanting!