What do you have now and how large are you trying to print?
Most modern DSLR's can print out to at least 11x13 so I doubt you have a camera issue, it's more than likely user issue.
Technically you can sell a photo taken with an iphone but this is MUCH deeper than your question "what type of camera are you using to take...", you need to consider what you are shooting, glass, ISO, and money to name a few things.
I shoot Nikon so I'll let the Canon guys weigh in on the gear but I don't think you are asking the right questions here and it may be in your best interest to spend a day reading this forum, allot of the answers are already here.