Hi dejapooh,
I'll try to answer your questions to the best of my ability.
First, Auctiva is free. There are no hidden fees to use the service. There are many benefits to it due to the features that are included which eBay charges extra for if you were to just list through eBay. A perfect example is multiple photos. You can have up to 15 (if I recall) photos in your listing through Auctiva at no additional charge.
Your Auctiva account is linked to your ebay account. So you can't have multiple accounts configured in your auctiva account. You would have to have separate Auctiva accounts as well.
I'm not sure what you mean by a built in sold feature. But there is very limited reporting capabilities in Auctiva right now and no accounting functions at all. While is can handle chaeckout functions it does not keep any records of sales figures etc. That would still need to be brought in from eBay and/or Paypal.
One thing to keep in mind about Auctiva is that it is a free service. And while it has improved greatly over the past year, there are still times when the system hiccups and you may see listings not post etc. It's usually a minor inconvenience but it does happen and it's something to think about if you are looking for 100% reliability. Other than that, you can't beat what you get in terms of features.
Good luck!