I've wondered the same thing. I've bought a lot from people of this site. I've also gotten a lot of incorrect plants, plants with lots of algae when they said there was none and poor packaging. Most of these are from the major sellers on this board. I hate to say it, but I wish I knew a good source.
Moss is the worst. Buy a moss type on here and you will get a ball of 5 types of moss mixed in.