In my experience, the fastest way to turn a profit is to find stuff on 30-40% off on clearance and sell the whole set new MISB (mint in sealed box) at less than retail, if possible. Otherwise, you end up selling just bits and pieces of your inventory while the rest sits there, profiting you nothing.
I aim for larger sets when doing this, as your profit is more substantial that way, though it takes a larger initial investment. For example, I bought 5 copies of Helms Deep for $90 each and sold them for $100-110 pretty quickly. Total profit of somewhere around $60. At worst, you know you can sell them almost instantly for what you paid for them if you buy them for 30% off or more, so you at least wont LOSE money that way.
Be prepared to do a lot of international shipping if you go this route. Even with my limited number of sales, I've sold sets/licensed figs to Hong Kong, Austraila, Spain, Portugal, and Brazil, among other countries. Making sure you are covered for shipping costs is vital on these sales, as shipping can run you over $60 for large sets (like Helms Deep to Australia).
All that said, the most profit can be made by parting out the entire set into its individual components, but you then have to sell ALL of the parts to make that work, and that's often difficult to do, especially for a new store with limited inventory, and it takes a long time before you'll realize a profit.