If you're a non-member, buy cooked lobsters for like 200 each and sell for 250 each? I used to do that... Or if you have high enough smithing, buy coal for 150 each, iron for about 90 each and then smith them [this may take a while] to get steel bars and once you're all out of coal and iron, and you have all steel bars, selling it for like 600 ~ 650 each.. That's about 210gp profit per bar AND some smithing experience.
If you're member.. I have no idea, maybe buy flax, turn into bow strings, then sell that? Or you can merchant sharks...
That's just some opinions from me..