I think that would be a great idea. Omeka’s focus on serving public cultural heritage organizations does mean that its strength of metadata doesn’t get a lot of use in for-profit areas. Which is too bad, because we do like to see artworks of any form supported and out in the world.
I don’t see any reason that a ShoppingCart plugin for Omeka couldn’t be built. Similarly, a Bidding plugin might also work well with it, depending on what kind of site you have in mind – I’ve imagined both possibilities as I’ve spoken with artists here, but never gotten so far as building one. It would be an interesting and appropriate new use for Omeka, to my mind.