Flippa is decent, but there is a lot of garbage to wade through to find anything real. Empire Flippers is a little better, they vet the sites some before posting them. Bizbuysell.com is mainly focused on brick and mortar businesses, but there are some online businesses for sale there as well.

Having said that, finding a true "passive income" business that is profitable and someone wants to sell is challenging. Most of them (that I have found) end up being scams. Do your due diligence and scrutinize their income and expense reports, traffic figures, etc. I can usually "smell" when something isn't right.