These are just my opinions as always and as always should only be taken as such. As johndevane suggested, my guess here would be that it's NOT the camera's fault. Based on your response to john's comments I don't know if this is what you "want to hear" but unless there's something physically defective with your equipment, 9.5 out of 10 times where there are issues with sharpness, focus, color, composition...it's almost always user error.
Now I have to say here that the first thing that caught my attention wasn't the focus as much as the color of the image. Looking at it closely, the guitar stand looks "white" but the rest of the image looks to have a rather yellow or greenish tint to it...not sure if it was just the light you were shooting under or what. Looking at the concrete in the driveway for example, concrete is usually gray and not yellowish/green. If you use Photoshop, take a look at the color balance under "Info" and you'll see that your colors there are indeed out of whack.
Now as far as the focus goes, I'm tempted to agree with bill boehme here in that it could very well just be a DOF problem...as in too shallow. The focus isn't really that bad and probably could be sharpened enough for Ebay pics in PP, but as bill suggested, next time I'd use a smaller aperture for greater DOF so that all of the instrument is in focus.
Also as others have suggested, a tripod (or other stable platform) is pretty much essential for any work of this nature. Camera shake can and does occur at 1/500 of a sec and under the right circumstances, it can even occur at 1/1000 of a sec, 1/2000 of a sec and so on. Again here, it's simply a matter of using your tools correctly.
On top of that, I would also strongly suggest a better background if you are trying to sell these instruments "professionally". You said "Honestly, I'm more concerned with getting optimal shots of the guitars themselves, rather than worrying about backgrounds and settings. If the background just happens to look good, then that's just a bonus" but that is not a professional approach at all. Remember actual retailers will often spend a great deal of money on the photography for their advertising. As bill stated, these image really just look rather ordinary...like average snap shots of a guitar turned backwards on a stand taken in a driveway. Here I would suggest browsing images of guitars in magazines such as Musician's Friend, AMS, Carvin, etc., etc., for some inspiration. "Product photography" is a very different beast from fine art or photo journalism. It's a very different approach...and that's basically what you are doing here is product photography. -If it where me- and I didn't have access to a proper studio with several flashes, backgrounds, a large light tent, etc., I would get some paper or even some large pieces of construction board taped together (you could clone out any seems later) and shoot in Northern shadows for a simulated softbox look...it actually works very well if you do it right.
Considering your response to johndevine's comments appeared to be a bit defensive I'm not really sure how your going to take this so again please understand that this is just my opinion but again since you asked the question "Is this my equipment's fault, or mine?" again I have to say it's probably not your gear at all. Everything I'm seeing here appears to be user error either in the setup and taking of the shots or in pp.
Peace,
Jim
"It is horrifying that we have to fight our own government to save the environment. " - Ansel Adams
Walczak Photography - www.walczakphoto.izfree.com
Gear List