Marketing can be very risky, you need to know what sells fast, and what does not. Mostly, cheap and seemingly worthless items will sell the fastest i.g Chronos, Orc, Gladiator. How fast you will be able to sell an item, should really be a factor when buying more expensive items, it should play a part in the cost of buying and selling the item.
Regarding the "I got 30k in the bank"... Do not, DO NOT, spend that 30k on one item. You need quantity not quality. So usually the best stuff, is the cheap stuff. Try buying full cheap sets, like Orc, and sell the items individually for market price.
Be committed. If you get lazy, then you won't see huge results, the pace is up to you, but the more committed, the more TC you'll be racking in.
Finally, the best tool you need is common sense. If you're not buying off the fourms or market, use safetrade. Don't trust people.
Hope this helped, and good luck!