Assuming your merchants 1) use web analytics, and 2) are honest, you could simply 'tag' people coming from your site in a way that your merchants will see in their analytics. In this way your merchants will be able to see how many tagged people went through their funnel and made a purchase. To tag people you can use Urchin Tracking Modules (UTM).

Here's Google's UTM creation tool:
https://support.google.com/analytics/answer/1033867?hl=en

Without UTM codes, once people get past a landing page, they all look the same, and are indistinguishable in analytics (i.e. useless for what you want). With UTM links on your site, your merchants can track people coming from your site all the way through their funnel.

It tracks returning users too, so even if they don't buy on their first visit to the merchant's site, they should still be tagged if they return and purchase later.