Long term trading profit is all about a sharp knowledge of the market and having the flexibility to meet demand. At it's lowest level, trading consists of buying low and selling high in a single station or solar system. More advanced trading includes playing the markets in different regions, hauling to meet demand and keeping an eye on general item trends. Then you get the high end stuff like market manipulation, cornering a market in a particular region and strategic trade hub trading (a station close to a large alliance or corp in nullsec).
Now I'm new so I won't even pretend to know how to do half the stuff I stated above. Though I will tell you whats working for me...
The 10's of billions of ISK profits you are talking about can only be achieved in the long run. How much you make in trading is always limited by the amount of capital you can put up and the turnover time from buying to selling. If the general product line I'm trading in gives me 20-40% profit on invested capital then, the size of my wallet will determine my profit in the end, how quickly I can make that profit depends on how efficient I am at buying, hauling then selling in another region. Higher capital = higher profit.
Use my example: I have set up a trading alt in each of the major trading hubs in EVE (Jita, Rens, Amarr and Dodixie) with skills in Broker Relations and Accounting (for those pesky taxes). In addition I have 2 haulers working towards freighters for hauling and flexibility. In a fairly short amount of time, i've been able to increase my starting ISK five fold, simply by buying from low buy orders in one region and selling to higher buy orders in another (This is a great way to increase turnover time on products when you're aiming to build up your capital reserves quickly as selling through sell orders typically takes longer as you're constantly waiting for buyers and being undercut by others). Once a week I'll scan through a couple of select products for price differences between markets, put my buy orders in for various regions then haul to more profitable regions 2 to 3 times a week to make my profits. Log in 2-3 times a day to update your buy orders (about 30 minutes), hauling 2-3 times a week takes 30-45 minutes a haul and the once a week scanning of markets takes roughly an hour... so the time investment is pretty small in comparison to missioning, mining, etc.
I currently make about 20-30 million a haul... this doesn't seem like much but like I said it all depends the size of your wallet. 2 weeks ago I was only making about 4-6 million a haul... and in about month I hope to make 200-300 million a haul. As your capital grows so does your potential for profit. From what I've seen trading has the potential to make you the most ISK/hour of any profession in EVE (but then again I'm a noob so I can't really confirm that).
The world of trading is far more complex and layered than the simple system I'm running above. The ISK potential is huge and the further you delve into this aspect of EVE the more interesting it gets. I would say give it a shot if your goal is the same as mine: Make enough ISK in 1 week to pay off all my accounts and fund my PVP main or nullsec corp with all the ISK it needs for ships, fittings, POS modules, etc.
EDIT: Just wanted to add a few pointers on increasing overall profit and efficiency:
1) Turnaround time from buying to selling is important, with that in mind try find products that trade in high volumes on markets... theres no use making 20 million ISK on buying and selling an item when it takes you 2 months to do so. Not only are you making painfully slow ISK, your funds are tied up and you'll lose all out on all that ISK you could have made with that chunk of capital
2) Deal in products with stable prices initially until you feel you have gained enough experience to start speculating on the items with wild price fluctuations. If you hit a product on an upswing you can make killing, if you catch it on a downswing you either have to sell at a loss or hold the goods until the next upswing which means tied-up capital and oppurtunity cost
3) When you're buying -> hauling -> selling, the % profit or straight profit figure is not always the best indicator of the profit efficiency of the product. Turnaround time (see point 1 above) is important as you have to factor in the hauling, but so is profit/m3 of your hauler. Sure buying a reselling a frigate for 50% profit but that could take up 25% of your cargohold when hauling meaning you're not making optimal use of your cargohold. eg: 8 000 000 profit @ 2500m3 of cargo space = 3200 profit/m3 vs. 50 000 profit @ 5m3 of cargo space = 10 000 profit/m3
So as a rule of thumb and if we assume there are no capital constraints from your side:
1) Single station trading efficiency is a function of turnaround time and profit so = profit / turnaround time = real profit per hour
2) Buy Haul Sell trading efficiency is a function of turnaround time and profit/m3 so = profit / m3 / turnaround time = real profit per hour
4) I just figured that I have so many more things to say so I'll rather write a guide on this matter given my own experiences in the past couple of weeks LOL
Would you mind sharing the link to the thread posted by derp? I’d love to take a look at it too. Thanks :)
http://skilltrainingcomplete.com/questions/1665/how-much-isk-should-i-be-making