I buy and sell skill-books, faction/deadspace/officer mods and implants covering all 5 major trade hubs (Jita, Amarr, Dodixie, Hek and Rens) plus several regions that don't have a major trade hub. I play fairly casually and update orders infrequently, often going several days between posting/updating orders. Some characters get updated more often than others and I update sell orders more consistently than buy orders.
In May I made 27 billion after expenses. Not bad, that's 3 billion less than last month but I spent less time working the market this month and I also spent nearly 5 billion extra this month starting up 6 new accounts.
That's my actual current total net worth, I don't pad the numbers by counting ISK I've earned but have since spent and no longer have. That'd be like calling myself a real life millionaire because I've earned over a million dollars during my life when the reality is I've never had anywhere near a million in total.
Next month will be interesting since my 6 new traders are just starting to go active after close to a month of skills training. Two started trading today, two more will start tomorrow and the last two will be active within a week. All but one of the new traders will handle skill books. Initially with only 125 orders they'll just do books valued at 5 million ISK (school price) or less but eventually they'll handle all skill books (unless I decide to leave the most expensive books with the original traders on each station). The 6th new trader will be taking over and eventually expanding the business my main developed on a fairly remote station over the last few months (more on that tomorrow).