Using Exact Target it is very possible to track conversions, using a basic img tag which passes some xml to the tracker.

You can read about how/what is required here: http://help.exacttarget.com/en/documentation/exacttarget/content/conversion_tracking/

Making this work with Google Tag manager is a bit tricky, but very possible

The process can be done in 3 easy steps:

1. Capture the relevant passed params from any cliked url

2. Pass that data to teh datalayer

3. Use the data in GTM

Google Tag Manager allows you to easily send all products to an external resource, by using the dataLayer, and a specific tag.

But what if you want to fire a tag only when a specific product, or set of products, have been purchased?

I worked out an easy way to get it done, using macro's and dataLayer.