Here are some pointers and what to expect.
1. I would definitely start selling on Ebay or Amazon since the sites are very popular and established however don't make any mistakes whatsoever! What I mean is that a few back to back mistakes will get you kicked off the site. My cousin was making $8000 a month on Ebay for many years and he took a small vacation, a few negative feedbacks came in (back to back) and he was booted. Poor guy lost 90% of all of his income.
2. Its a good idea to open a site but be prepared to work on it forever. You have to push and push and push. Each push will bring you an inch closer to your goals and pushing a custom website takes a LOT of pushes to make popular. I've pushed a few sites and got 2 to popularity level but each one took at least a year of pushing before it got to that point. The idea is to make money on amazon/ebay while your pushing your sites.
3. Be prepared for catastrophes. There are people who make $10,000 a month and lose it all over night. It happened to me once when I was at 15k a month. When you lose it, its gone. It can happen for no reason at all! One dude had a squidoo empire where he spend every day writing articles and the whole thing sunk with NO explanation as to why he was shut down. Another guy lose a million dollar Ebay account. It CAN happen so don't put all of your eggs in one basket. Spread it out as much as you can.
4. Pay your taxes asap. I understand that some people have children and are making $500 a month but you really have to get into paying taxes asap to avoid future problems. For example, some IMers make $5000 on year one, $50,000 on year 3, then all of a sudden they make $150,000 on year 5 but they never paid taxes and it can become a problem. Paying is your best bet even though it sucks.
5. Always check your emails/phone calls every day even on Sunday. Buyers don't care about your days off. Keep them happy and avoid bad feedback.
6. Be prepared for lots of fraud. People will use unauthorized cards all the time to purchase. Learn about online fraud control.
7. Be careful when in a dispute with buyers. They usually in UNLESS you provide massive amounts of proof to back your claim. You will usually lose though. I am hoping this changes in the future.
8. Remember that is certainly is possible to make a 6 - 7 figure income every year but its not easy to get to that point. You have to climb the wall of chaos to get there. Just keep at it but make sure you are properly funded in the meantime for regular life stuff (food, rent). Don't just quit your job. Stay in your job until you have seen a steady income for AT LEAST 3 months straight.
9. Get into something you like. If you hate writing, don't go writing all kinds of blogs for an income. Do what you truly love and you will excel at it. There are tons of options so pick the one you love most.
10. If you want to really pursue this. Don't give up. Keep on pushing and building on 1 - 2 projects but I would recommend just 1. Once you get the 1 right, start a 2nd but don't overwhelm yourself.
This is truth of it all. Its not easy and it can be catastrophic. Many lives were ruined and fortunes lost but even then, some continued to pursue IM until they were successful. You can do it, but put your war-face on before getting into this.