Your software requirements will be driven by your photographic equipment really. Don't confuse the advice that myself and Murphy are giving you as being the same.
Murphy is giving you good advice on how to take a great pic up front, in regards to equipment needed. My advice is aimed at giving you options to make an ordinary pic look great when you are using just everyday photographic equipment.
Essentially, Murphy is speaking to pre-production methods and I am speaking to post production methods. You can obtain the same result with both.
Now if you are wanting to publish in a world class mag, then you need both equipment to give you the best pic to start with and then the best software to finish with, but you're not doing that.
You're talking about taking pics of second hand items for eBay, so you can choose one of the other, but you don't need both.
Good camera equipment, good technique and you will get good pics that will require little or no post production work. Average pics taken with average everyday photographic equipment will require more reliability on post production to create the quality.
You have to work out which best suits you, getting the best pic or making it the best pic after you've already taken the shot, but you don't need both for eBay.
In regards to seriously good software, hard to go past Adobe's Photoshop, but Corel have a decent enough editor called Photo Paint. Neither are cheap, and will require some level of knowledge in getting the best results from them.
That's no different than choosing to take the quality pic to start with option, still a level of knowledge there to obtain before you will turn out perfect pics every time.
It's comes down to what makes you more comfortable I think. Personally, I'm very much into editing because I enjoy the manipulation side of it, where as you might be more into the up front artistic side in getting the right shot up front.
That's a choice you would have to make.
The software programs I have suggested are good and as such are costly, but there are plenty of options out there, some free.