Ebay seems to change faster than then the price of gas lately, but a few months ago, it was definitely wise to list a few auctions so your items are "searchable". Follow Debz advice and lead folks to your store from your auction. A smart thing I've seen folks do is to offer a shipping discount for anything bought with Buy It Now in the store. Just a thought. More info on searchability of store items:
Items in stores did not return in a search unless:
1. The person selected to include stores in the search (most buyers don't know this.) or
2. When 30 or fewer of that item are available on Ebay (not going to be the case with tshirts.).
Okay, just went to Ebay and checked, it's still the same. Here's a link to the page on Ebay that explains how store inventory is not returned in the main searches. Read this page very carefully.
How Buyers See Your eBayStore Listings