One thing you need to remember is that while the work here that you're buying is vector graphics (good thing), the images you see are uploaded as bitmaps (jpg, png, or gif) and the site compresses them a little bit to. So what you're seeing is a little different than the true source files you'll end up with after the contest is over.
But I can't explain why some of the entries in this contest are almost blurry, if not kind of fuzzy.