Many sellers are new, or just forgot. Some don't actually know what the shipping cost is until they put it into a box and get it weighed. Of course, there is the occasional rare jerk that wants to charge $10 shipping for a $5 item. One could deduce that the more 'popular' sellers out there just assume that their shipping is the same as their many other items, which a person may not run across, especially, say, if "BillyBob12321" with a rating of 925 sells primarily flowers and flower accessory, but had to get rid of a video board for a computer on the side. Unless you were a rampant flower shopper and decided that you wanted a new video board, you really wouldn't know what this person's shipping is.
Another theory is that the seller only wants 'established' consumers, being those who have already proven that they have forked over the dough on time and under his terms. This keeps any possible NPB's out of the equation for the seller, but raises questions among the 'have-nots' about this person's real credibility.