DSLR photo development?
Where is the best place to develop clear/sellable photos from a digital SLR camera (Nikon D5100)
You do not "develop" digital files. You PRINT them. FILM is "developed".
There are tons of places to have photos printed from your camera. Mpix is an excellent lab. Whether any of your photos are "sellable" is an entirely different matter.
In Lightroom.
The whole point of digital photography is that you don't have to develop it!
If you mean where to print, it's good that you know that professional images should be printed by a professional print shop as oppose to at home. There are an insane array of options for printing photos and there's no such thing as "best" (there's no such thing as best anything in life without a purpose). Clarity and sell-ability have much more to do with the quality if your work than the printer. MPIX is a good consumer grade print shop with a few "professional" options. Professional print shops are usually locally based but this is changing as people are able to move files over the internet. I use a place called Nation's Photo.
http://www.nationsphotolab.com/
A good rule of thumb as to if a printer is professional or not is if they allow you to submit TIFF files instead of JPG. A very high quality printer can also do chemical/optical prints, not just ink prints.
Beyond that it comes down to the number of choices and sizes of paper available and how good their customer service is for ensuring correct calibration. By the way, is your monitor calibrated correctly? Else you will be getting prints back that look very different from what you expect. Again, I have to emphasize that clarity and sell-ability first come from YOU. In order to make professional quality prints, one must first be a professional. It's not simply a matter of handing things off to someone else and crossing your fingers.
That being said, the D5100 is a consumer grade camera with a small sensor and you are likely using a garbage kit lens. It's also critical to use editing software correctly so that you produce high resolution, high colour depth files. So while you are not a professional, you can begin to educate yourself about printing and making higher quality prints now. And that's very good. You're off to a great start! Now it's just a matter of sitting down with a book about how to print digital photography and experimenting by making numerous prints of the same images so you can begin to understand how different types of ink, chemicals, and paper affect your image.
Best of luck!