Gotta start small and build some momentum - sell poly bags, part out some sets, or even stuff lying around your house to get at least 20-25 feedback before moving up to small sets. Then, when the time is right, mix in some big sets. Doesn't hurt to buy things to get feedback too!
Find some great eBay lego sellers and copy their descriptions - don't actually copy them but mimic their detail, policies, and style.
There's also plenty of info on this site, and others, on how to be a good seller. Just takes some time and a browser to read it all....the trick is to actually follow through.
Remember, it does take work - there's good money to be made, but if it was a walk in the park, then everyone would do it. (See other msg board threads on speculation of the golden age of reselling....)
Good luck!