Here are a couple of good comparisons between reputable breeders and backyard breeders:
Dog-Play: Use this guide to obtain a quality puppy from a responsible breeder Backyard Breeder vs Reputable Breeder
You brought up a very good point. Breeders who advertise here are not screened or approved by YT in anyway. The only requirement for selling puppies here is that the puppies be at least 10 weeks old and if the seller wants to post a link to a website, that she be a donating member. A potential buyer should never assume that just because a puppy is advertised for sale here, its breeder is reputable. It is very important to carefully research any breeder you are considering.
There are some great guidelines posted in the For Sale section:
Yorkies For Sale or Wanted - YorkieTalk.com Forums - Yorkshire Terrier Community