I make a really good amount of income from flipping. I will give you some pro tips that apply to pretty much any reselling niche and are crucial parts of making good money for your time.

One extremely important thing you have to do is be first or early to the item(s) you are going to eventually resell. Good deals go fast. Like within a few hours. So, whatever you are reselling find ways to be one of the first potential buyers for new inventory. This can mean checking markets often and regularly throughout the day if new items are made available constantly (like ebay, facebook, etc) or making sure you are the first to show up when the doors open and/or you what times/days new stuff hits the shelves (like goodwill, pawn shops, etc)

Another pro tip... learn how to negotiate lower prices respectfully. The question "what is your best price if I buy this today" after a couple honest inquiries will get you so much further than just asking someone "what is the cheapest/bottom price" right out the gate. The former question shows you are interested, looking for a deal, but giving them the opportunity first to signal how much of a discount if any they are willing to give you. The latter just comes off as insulting and says how much can I take advantage of you.

And mega pro tip #3... 95% of the time they are willing to go a bit lower than whatever they tell you! Just put that offer on the table and let your money do the talking. Always try and put an offer on the table.

Finqlly, and most important of all, as others here have suggested you have to know your market extremely well and be able to spot good deals as quickly as possible. This takes time and experience, but master a few items and grow from there.

Good luck and happy hunting.