I mean, there's always the D&W section:
https://www.blackhatworld.com/forums/domains-websites-for-sale.195/
Then there are PLR sites (although I'm not sure how relevant they still are today) like the ones I've mentioned in this post:
https://www.blackhatworld.com/seo/w...ts-for-sell-in-my-blogs.1469573/post-16037476
Besides Flippa, there's also Empire Flippers that I know is pretty big (not as big as Flippa I think, but still) and a couple more whose names I don't remember off the top of my head...
And finally, a quick google search found these:
https://nicheinvestor.com/https://outandbeyond.com/blogs-for-sale/https://www.premadeniches.com/https://loudsites.com/https://blogsbiz.com/
Oh yeah, and FB groups, too, these also used to trade niche blogs a few years ago... also, probably sites like Reddit might have some subs for this, as well...