This is not completely passive income, but after 1 ~ 2 months of some committed work, it can become passive income. This is assuming you know what you are doing and you have experience with e-commerce. (I was losing money for the first 3 months)

-Private labeling on Amazon using FBA (Fulfillment by Amazon)

I currently earn some passive income (~$1000/month) by selling products on Amazon. Amazon FBA is great because you just send all the products to Amazon warehouse and they will do the packaging and shipping for you. If you shipped out all the items yourself, it wouldn't be so passive. But since all my items are currently at Amazon warehouse and everything's being shipped by Amazon, I spend little time at the moment.

A summarized process will be 1. Sign-up for Amazon seller account 2. Find an item to sell on Alibaba 3. Purchase the item, receive, check, and send to Amazon. 4. Perform SEO for your product page 5. Repeat 2 - 4 6. After some success, switch to Amazon Seller Merchant Pro.

The most difficult part wasn't finding the right item, but rather promoting the page. Ranking my product will involve tons of keyword research, pinning everything relevant on Pinterest, and revising copy-writing.

Once I start selling about 10 products/day for a single item, I'd move on to a new product. The best time to start probably is NOT at 3rd or 4th quarter. You want to have steady sales by 3rd quarter. Because if you do, you will experience some shockingly crazy sales starting November lasting until early January. If I now sell 10 units/day for product X, I was selling 100 units/day starting late November. I regret not stocking more items, but it was my first year doing this and it's a lesson learned.