Last week, I had a conversation with Rozelle in South Africa about why her photos were not as sharp as some of her local competitors. She was ready to get a new camera and lens but after some discussion about her workflow, it turned out she was bracketing but not being diligent about ensuring the camera didn't move.
I think what you will find is that even if you shoot with a great lens and quality DSLR camera, images need a little sharpening for the specific device they are being displayed on. For real estate images, that device is a computer display. Images intended for printed media require different sharpening than a computer display.
Sharpening is a fairly complex subject and is related to noise reduction. Anthony Morganti goes through this in the accompanying tutorial. Anthony's tutorial covers the basics for any image but it all applies to real estate work. A classic resource for sharpening is Real World Image Sharpening with Adobe Photoshop, Camera Raw, and Lightroom, by Bruce Fraser and Jeff Schewe.
Here are some sharpness considerations specifically related to shooting real estate:
What do you do to keep your real estate photography results sharp?