Building a pay per transaction site

I have to build a comparison shopping web site,
where I will be paid by merchants on per successuful transaction basis.

what are the ways to track whether a successful transaction was initiated from my site?

Are open source solutions/templates available for the same?

Does any CMS(for e.g. Joomla) do it?

In summary, let me know the free/paid solutions of the above problem.

thanks in advance.