Dont crop too much !!!
As a Graphic Designer, pictures are for me a very important asset. That's why sometimes I feel frustrated when I find the right picture for my design, but I can't use because the photo has been heavily cropped.
A good example is a picture of a model face, where the head (and even the chin sometimes) has been cropped to make a close up of her eyes.
Pictures like this cannot be used, because I can't isolate the subject to use it in a design piece, unless I blow up the picture in order to make it bleed out the design border. The same situation happens with cropped shoulders, where I'm forced to put the subject in the lower left or right corner of the design, to don't make evident that I don't have more picture to work with.
Just take in account that those beautiful pictures you are selling usually are not the final product (i.e: it is not going to be printed out as is in a 4x6 photo paper), BUT ARE TO BE USED AS A PART OF A DESIGN PIECE.
I'm not saying that you shouldn't crop, but also provide an alternative non-cropped version of the image to allow the image to be used in a wider variety of works.