Hi. As far as I know, it's not possible from eBay to direct buyers to your Auctiva store.
Well, that said, it's suppose to be okay to put the link in your listings. I've read of people having their auctions canceled for link violation, which is just wrong.
I haven't wanted to risk it, as someone doesn't like me already and reports me for who-knows-what. Ebay yanks them anyway. Thankfully, it hasn't happened in awhile. The plus side is that some of the eBay folks now know who I am, so I can get help on things that used to just be a total waste of time. lol
You can certainly put links on your ME page. As far as the permanent links from eBay directing shoppers to your store, those can't be changed.
Keep your keywords updated and you'll be found on Google for sure. I'm even getting hits from a site I never heard of that brings in a little traffic and am thankful for all I can get. I'm sure it's a result of using Auctiva because I haven't done anything to get listed on that site. :-)
Hope this helps and sorry for the OT rant. ;-)