It would be helpful if this was explained at the outset.
I'm afraid that the information is widely posted on these boards.
A domain name is not a prerequiste to an Auctiva store as it is only another window, lets say, on your eBay items, however using it with the Auctiva tools and image hosting is extremely cost effective when you become practised in using them.
If you believed you were misled I would suggest you contact Auctiva support before you start using it and explain, they may just cancel it for you, depends on the small print. I preferred to buy a domain name from outside because I beleive Auctiva retain ownership of the domain name they sell and so it cannot be used for any other purpose. Perhaps a regular here can confirm this please.
Alternatively I suggest you start a new thread to ask others how to use the domain name effectively.
Since I last wrote on this thread I like many others have had enough of eBay and am now selling 99% of my items elsewhere so I cannot really advise much further.