When it comes to deciding to sell prints, I would assume a prime concern would be the clarity of the prints in question. There are several things one can do to ensure that larger and larger sizes still yield the quality essential to making the sale.
One reason professional grade cameras are so coveted is that you can determine the size and quality of the photos you take. For example, on my Nikon I can choose between small, medium, and large sizes and basic, normal, fine, and RAW qualities. Since I haven't graduated to working with RAW files yet, I leave the selections set to large and fine. This ensures that the dimensions of the pictures I take are appropriately huge (3872x2592) and that when it comes time to print, I can go up to a 20x30 poster if I wish.
Now, the whole mumbo-jumbo surrounding file sizes and print sizes and ppi and the link between all of that is still a little fuzzy to me, and I'm not going to dig myself a hole I can't explain my way out of. There are lots of great articles on the web by people who print pictures for a living who can tell you all you need to know. Here is what I can offer today:
One of the things I always, always do when reviewing photos after a shoot is magnify them in my photo editor to see just how clear they really are. A picture that looks great can actually be hiding blurred edges that aren't an issue until it comes time to make a 16x20 and you're left with egg on your face. Take these pics from my trip to the Butterfly House as a prime example:
Both look crisp, clear, and make the butterflies look really great. But am I ready to blow the pictures up and sell them? Let's take a closer look.
In the first picture -
Aha. Not so clear after all. And you may be thinking, well, it's not so bad, it's actually a very small part of the picture when you take into account the whole.
But check out the same zoom in on the second picture:
MUCH better. Why settle for the first when you can be assured that a picture retaining so much fine detail on the micro level will just give you even better clarity and quality overall?
And there are things you can do while taking pictures to ensure that you get the best clarity. A big one is using a tripod; it reduces the amount of shake you normally produce when hand-holding your camera, and also makes it much easier to take low-light shots. Another thing is to shoot in as much light as possible if you don't have a tripod. I've taken pictures out of the windows of moving cars that are fabulously clear, because there was enough sunlight to allow for a quick shutter speed, and I also use a lens with vibration reduction to reduce the effect of any shake even more. As I mentioned in my zoo post, if you're shooting a moving subject, try using the sports setting.
So, the moral of the story is, do everything you can while taking pictures to keep them clear, and then always magnify them to check afterwards before you start making and selling larger prints.