I have a basic eBay shop. Requires about $16 a month.
Basically the differences between having a shop and not having a shop:
-You get the possibility of uploading 12-free pictures on your listings with a basic shop.
-You get a discount on listing fee.
-A selling manager will be included in your 'My eBay', basically the whole layout changes and there will be many more features. But the simplicity of eBay will be gone.
-You will see everything related to your selling at a glance:
The amount of sales last one day, last 7-days, last 30 days, last 90 days and last 120 days. You'll have the seller dashboard. Alerts - New international sales (number).
You'll also see the listing actvity like active listings, ending today, format, schedules etc..
All this on one page is very informative which was not the case when I did not have a store.
Marketing tools - You can send out emails, you can also start discounts on selected categories.
In short, the looks of your eBay page will become a little more professional, get marketing tools, use the store to market your other products below your listing or above, the possible discounts on listing fee. More detailed explanations are available at eBay site.