I think it's critical to know exactly what your buying when dealing on CL. If you're buying new, unopened sets, check box condition and seals and know what they're worth (S@H retail and big box store retail). Your MF purchase is a perfect example - you can find this set for a discount (online or B&M), and can likely get a better deal from a TRU BOGO 50% or WM/Target clearance. These sellers allow you to return the set within a certain time period, making the purchase MUCH less risky than a CL sale. How much that risk is worth is really your call.
If you're buying lots to piece together and sell on eBay/Bricklink/Brick Classifieds, you have to know EXACTLY what components in the set comprise the most value. Typically, these are minifigs, but many times there are unique or rare pieces that will cost you an arm and a leg to purchase that will really sink your investment if they're missing. Also, stickers and instruction manuals are key components that can be expensive to replace if they're not there.
Like anything else, those that do their research, know exactly what they're buying, and can accurately forecast selling prices will make money. Good luck on your investing!