This can be done with Pay per action and referral links such as site X has sent us 1000 users and 10 of those have signed up so we pay them for these 10 signup

This can be done with Pay per action and referral links such as site X has sent us 1000 users and 10 of those have signed up so we pay them for these 10 signups/purchases whatever interestingly amazon are the leaders here...