Sorry, but I just noticed that you said you bought your cameera used. I think the problem is going to be with the camera and I think it is going to be that someone tried to save some money on pec pads and cleaned the sensor with cotton swabs. It would not be the first time someone has done this, it wouldn't be the first time I have run into it personally, and if this is the case, then the sensor shield needs to be replaced.
Get your focused checked, google "focus test chart" and you should be able to find something, or just use a ruler. I had my camera calibrated and it worked wonders. If you don't get sharp results from any lens it's probably your camera.
Also you say your camera is second hand, how many exposures have been done with it? Sensors deteriorate over time getting softer and softer.
Looking through your gallery, I think you have plenty of sharp images. Not pro-grade sharp but to achieve that you need pro-grade gear and editing techniques.
I shot a pic at a ruler the other day, I had it on 1.8 to get a narrow field and shot it with aperture prio (and everything else on auto) on tripod and it seems to be right where I focused but "dreamy" even in the "sharper" parts.
The problem being that the photos I have here usually 1-3 taken out from 60-70 picture in a session which I have to throw away due to to sharpness - I want to fix that because it's frustrating.
Unsure how many exposures it's on my camera but the problem was when I got it which I then thought was because I was new, but I still have the issue (got it in 2008) and I don't know what I'm doing wrong.
I don't need the crisp sharpness, I just need things to be less dreamy in my product shots