First tip, Don't expect to buy and immediately sell for a profit. You'll need to buy, hold, (BE PATIENT), then sell. The usual progression is:
1) TRY to buy near the end of an item's lifespan. This requires research, and nobody truly knows when an item will retire, its an art not a science. Typically 2 year lifespan for products, but it differs GREATLY from set to set,
2) TRY to buy on discount. (30-50%) Never pay Full Retail unless you already see the product has retired and is actively bringing big profits.
3) Decide what your needed profit margin is, and stick to it. Don't get greedy hoping for an item to continue to rise. Sometimes they plateau, sometimes they come crashing back to earth due to a re-release (Architecture studio, and Minecraft are examples)
4) Sell on ebay with great pictures, buy it now / best offer, don't accept returns (scammers abound), and try to stockpile shipping material AHEAD of time so you're not scrambling when something does sell. - Just my opins, many may differ.
Finally, welcome! There's plenty of discount lego sets to go around (unless you're talking Amazon Warehouse deals). Hope you get to contribute a few tips along the way.