Well, in this part of the question you are right. Many sellers, getting a good rating and a large stream of orders, relax and approach their duties less carefully.
There are situations even worse - scammers first make themselves an excellent rating by trading themselves at a loss, and then they deceive a huge number of buyers.
Yes, I completely forgot about this option, although personally I came across this. I think the blockchain will help with protection from various kinds of unreliable sellers.