I would do obby capes and black masks to start until you learn the barrows market. Then you can transition into the barrows market pretty easily. While you're merching obby capes and black masks be on the look out for cheap obby mauls and obby shields. These were my buying points/selling points and I made a good 40M legit while having fun
Obby shield: I bought for around 150k, was happy to sell at 200k but often sold at 250k
Obby maul: I bought for around 140k, was happy to sell at 180k, often sold at 200k
Obby cape: Bought at 260k-280k, sold at 300-320k (depending on other merchants on at the time and what not)
Black masks: Bought at 500-550k, sold at 650-700k'
Zyb3z will be your best friend, so ALWAYS have that open and trades going on in that screen too. If you're lucky you can snag a few hundred glories for 60k and sell them all for 75k easily. I operate on a 50k rule, if I can make 50k off an item - I buy it. If it doesn't make me 50k, it's not worth it. If you have any questions at all man I'd love to help you out just shoot me a PM
edit: I really would stay away from d hide, runes, rune items, and ammo as the profit margin is really not that great. HOWEVER, dwarf cannons work well. You can pick them up for 600k and sell them for 750k at times