1) Don't get into group breaks of modern product or bust cases. Good way to go broke fast. Sure, some people hit big, but they do in Vegas too. Modern card buying is like buying lottery tickets.
2) Read my primer on COMC here:
https://www.blowoutforums.com/showthread.php?t=1211826
3) Buy low, real low. Don't buy gigantic lots and pay market price. Make sure you have enough meat on the bone to not get eaten up by eBay fees, shipping costs, etc.
4) Make sure you know what your cards are worth, and where best to sell them. eBay is good, but if you don't describe your cards right, you'll just be feeding the flippers on eBay and not making the money for yourself.
5) Read the COMC flipping thread:
https://www.blowoutforums.com/showthread.php?t=807253
Shows how some can take cards and turn them into profits by finding diamonds in the rough, see which cards sell for premiums, etc. I am a fan of Topps error/variation cards listed in PSA master sets.
6) Learn how to grade cards, learn the graded card market, and send your submissions to PSA on bulk specials.
7) Figure out how to tell real cards from reprints.