When I buy Stock images, I want perfect (or nearly perfect) visual and technical quality. No blurred edges, no soft focus, vibrant colors, proper white balance and good details for use in commercial projects.
As a visual training exercise, look at old magazines and cut out your favorite images. If you don't have any magazines at home, old magazines are cheap as chips at thrift stores and garage sales.
What do commercial images have that yours don't? Is it composition, depth-of-field, lighting, color saturation, details? Make notes about what you like and don't like. This little visual exercise will help train your eye on what to look for in your own photos and hopefully make you a better Stock photographer.
Also read your Stock Contributor User Guide. There is a lot of good information there to help you succeed at Stock.