I’m making around $1,500/month of passive income. Here is what it worked for me: I created a web tool for stock technical analysis but instead trying to acquire clients by myself, I contacted some players with a solid distribution channel who lacks some tools to sell. So I offered them white label to sell inside their website, splitting the profits by 50/50. No cost for me to acquire users, sales, marketing or even support. I believe it can work with any kind of product. Pretty much the same as selling your app in a marketplace, but I would say it is easier to sell having a brand on top of it.