No, since you're only selling a few items here and there, a custom domain would probably not be of much benefit to you. Custom domains are more beneficial to sellers who always have inventory in their stores and are looking to promote them whenever and wherever they can.
It really doesn't have anything to do with keywords either. If you were to purchase a custom domain, your buyers would be able to reach your Auctiva store by entering http://www.kimmysstore.com, or whatever available domain you choose, into the URL field of their browsers, rather than your default Auctiva store URL, which would be something lik http://store.auctiva.com/kimmy.
Hope that makes sense
-Mike