Lots of good advice here so far. I'll try to summarize from my point of view
set a goal, make a plan, and then stick to it.
when I started out, my goal was to subsidize the lego hobby. I would buy 2 at discount and quick-flip one for ~50% gain to subsidize the one I built. Or sell off minifigs and keep the bricks to make the bricks nearly free.
later, realizing that better returns could be made with a bit longer hold, I dropped both of those strategies and started buying in bulk. My goal changed, and thus so did my plan.
don't spend money you can't afford to lose
I think the strategy of keeping your lego funds in a paypal account is quite good. This will put a natural brake on your spending, and trigger the pain center of your brain if the funds get too low. Credit cards reduce the impact on that pain center, and make it easier to over spend (on both lego, and life).
always keep in mind that buying is easy, selling is hard. Don't buy more than you can afford to store, or have time to sell.
When you're starting out (say for the 1st year or 2), its a good idea to keep all of the funds from sales in the account and add to it from other sources when you can.
This is the building phase. Once capital is built up, and your buy/sell process is working smoothly, then you can draw funds from the sales---i.e. reinvest 1/2 and pull 1/2, depending on storage/time constraints.
Hope this helps.