You can make 300 plus in one weekend hitting up garage and liquidation sales, then selling the stuff on eBay. If you go on letgo, there are people selling pallets of electronics for the low that you can flip on eBay as well.
This weekend, I'm going to try to find out where I could go to find the warehouses that sell pallets of Amazon Returns for the low. It's a lot of side money out here.
To turn it into truly passive income, you can send it directly to a fulfillment center, anytime somebody orders some of your stuff, they could ship it off for you for a small percentage of your profit