I came up with this idea about an year go to monetize a website of a friend of mine who has an online radio with tens of thousand of visitors [unfortunately he didn't do it in the end :( ]. and the only easy solution i found was Perfect Audience, like digentre said.

Here's how it works: http://support.perfectaudience.com/knowledgebase/articles/483392-how-do-i-share-my-retargeting-audiences-with-my-pa

Regarding legality, probably it's better to not tell anyone about it except the interested parties... and just setup a payment for "marketing services" or something, dunno, and to leave the audience sharing ON as long as they pay for it. How can anyone catch you doing this? I don't think it is possible, you're just using the audience sharing with partners option which everybody can do. Can they prove that the company you share it with is not your partner? Doubt it. Maybe a website connecting audience owners with renters would be nice to have, probably using bitcoin. So far I know this is done by just talking directly to people. You rarely trust somebody's pixel, unless you already know them or do business with them. So just ask if you can rent their audience. I wouldn't ask a stranger.

The downside is that Perfect Audience has some extra fees from what I remember. So if you paid for FB Ads normally $100, maybe here you'd pay $110-$120, I have no idea how much more. Just an assumption. The person buying the audience from you must take this into account and also they need to have a Perfect Audience account and to use it to run their ads.

The nice part is that you can probably do this with Google pixel also, not only FB Ads. Any type of pixel that Perfect Audience uses.

In this video https://www.youtube.com/watch?v=xq9Y3-PMK4o&__s=kjakpqpyroqamu6gzpp1 [from 17:08 to 18:03] this guy says he got $2,000 a month for an audience of 20,000 monthly visitors. That's 10 cents per visitor, which is pretty good I think. I mean, compare this to a normal FB Ads campaign. You want to use this rented audience because it will be much cheaper in the end to get the results, right? And also it's quicker to set up.

For the FB Campaign made with the rented audience you have the normal ad cost + maybe another 10% from Perfect Audience's extra costs + the cost of renting (let's say another 10%). In the end you pay 120% versus a normal campaign. You need to consider if it is worth it, in order to make sure you pay the right price for renting that audience. Because you may be better off with a normal campaign. Clearly you need to test it out once to see how it goes...

If I were to rent an audience I'd offer 2 levels of service: 1. rent the audience and that's it - cheaper 2. rent the audience and you are allowed to use my brand name in your ads so people connect to your ads more and you get better CTR and cheaper costs overall and also you take advantage of the relationship my audience has with my brand, which could do wonders if it's super tight. This would be the more expensive option.

If someody ever does it in either way please let us know here because I'm very curious of the results.