first, we need to clarify what an "audience" is...
an audience can be "interest based", a thematic approach to both positive and negative "audience" targeting (on the display network).
an audience can also be a "remarketing" audience, where you've put a defined AdWords remarketing code on a page, to tag people who visit that page.
so if you create a remarketing audience by putting the code on your receipt page, you are creating a targetable (positive or negative) "audience", and let's say you name that audience "already bought".
you can then use your "already bought" remarketing audience as a negative targeting audience for any campaigns in your account. to eliminate showing any display network campaign's ads, to those people who already bought, add the "already bought" remarketing audience as a negative audience.
once you start thinking it through, you can see the possibilities run deep... you might offer a coupon to first time buyers by negging "already bought"... you might offer special upsells to those same folks... you might target another very different product, because they bought and now know your name...
more strategy ideas here:
title: Strategies for setting up remarketing campaigns
the system has some holes (cookies get deleted, people can opt out), but these holes are a small issue compared to what can be achieved. more details about the tracking (and its issues) here:
in my head, i call it rethumping, not remarketing. :-)