You don't need a Store to sell on eBay. Monthly cost on a Store starts at $15 plus a fee for each listing.
You can list 50 auctions a month for free and there have been frequent promotions that allow up to 50,000 free auctions. So really don't bother with the option.
Stores are useful for experienced sellers with many unique items for sale. The minimum is usually given as 54 different items. Most Store owners are listing at Fixed Price /Good Til Cancelled and usually have several hundred items listed.
This chart should give you a better idea of whether a Store will fit into your business plan.
http://pages.ebay.ca/sellerinformation/news/FeeIllustrator.html