You made a nice photo, so always zoom in and see yourself if the details are also nice.
On the photo here, I see a lot of JPEG artifacts which result from low JPEG quality. Save JPEGs of your photo in quality 3, 6 and 12 and compare the details. Compare with a TIFF (all in unchanged resolution). JPEG 12 should show none of these artifacts; TIFF doesn't for sure.
Others mentioned high chromatic aberration resulting from the low quality standard zoom. You can correct it, like someone else said, in lens corrections. Look at the vertical edges with high contrast and correct until red and green fringes disappear completely and the houses have the colours they appeared in.
As a professional, consider using prime lenses or high quality zooms for better image quality.
Remove dust like someone mentioned. You can use a special tool or the stamp in photoshop for that.
Consider resolution: For printing, the photo should have 300 dpi. So calculate yourself which sizes shutterstock offers and which resolution is necessary. For 20 inch you would need 6000 pixel. If the resolution is lower, enlarge the image size. Try out the different methods for calculating the image in the dialogue: image size, and look if there's a difference in the details.
Try one image without sharpening, only correction of chromatic aberration, dust, any other mistakes you find or necessary corrections of overall colour, lightness, contrast etc., resolution, save as TIFF. Look if you like the details in the way that they fit the expression of the photo. Print the whole 1:1 in large size, or first in small details if you have a small printer. Look at them, see if you like them. Sharpening is not always necessary and can make photos look more artificial.
You made many things right like ISO 100, tripod, using a DSLR and above all making a nice photo of this scene.