Craigslist: Craigslist is definitely the scrappiest of the major online resale options. The advantage of Craigslist is its enticing profit potential, thanks to the total lack of listing and selling fees for most items. The disadvantages are many, but include potential safety risks and higher chances of nonpayment. If you do choose Craigslist, keep your wits about you and use the buddy system.
Matched Betting (not available in the USA) enables you to gamble (nearly) risk-free. I say nearly as there is always room for error! It involves placing bets on all outcomes of an event, using free online bets given out by the Bookies. Although it may take a while to get your head around the concept, Matched Betting can make a substantial addition to your monthly income.
You'll also need ecommerce software, fulfillment software, worry about warehousing, customer service, refunds and so on. But that's not all. You'll also need traffic. Think search engine optimization, Facebook ads, and other social media campaigns. Sound like a lot of work? Sure, it is. Especially if you do it all on your own. You could opt for Amazon's platform, which might be the easier route. But, then again, at the end of the day, this is a serious business, which could produce significant profits. So you're either all in or you're not.