Yep, I agree with all above, great tool. Actually, I would think ever serious eBay business would be hooked up to Terapeak. At this point there is no doubt you would be best off taking the free trial, but wait until you're ready to pull the trigger, or the information you gain from the trial may not be current when you do jump in.
Don't overlook free tools like eBay's own Pulse, and even sites like goofbay.com. Of course they won't deliver the level of sophistication that Terapeak does, but they won't cost you anything neither.
In regards to choosing eBay over another auction site, eBay will usually be the most expensive option for sellers, but they deliver the kind of traffic that no one else does. From a business perspective, you really need to be covering as much of the market as you possibly can, so even if you are on eBay, you shouldn't discount other auctions sites out of site.
It usually comes down to cost effectiveness when trying to cover a market, but there are plenty of free listing sites out there now, all with different levels of traffic to choose from. Now they may not all prove to be successful for you, but if it's not costing you anything up front apart from your time, then what do you have to loose by covering them!
An eBay store option comes down to stock volume for me essentially. If you don't have a lot of stock to house in a store platform, then I would suggest kicking off with the standard listing frame and saving yourself the extra outlay.
If you want to make your listings stand out, then there are a range of free templates to choose from, just Google those. If you are creative with some graphics work on the PC, then of course you can always design your own.