You're so right. My first experience of this was, as a complete novice about 10 years ago, being sold 3 blood red parrots, a common pleco and a 50 ltr (yes litre!) tank to house them in! Partly my fault admittedly for not researching first by my annoyance/frustration at this lead me to research everything and anything I could about keeping tropical fish to the point where I'm seriously considering setting up my own business. Whilst there's no exact science I detest suppliers who don't put first the health and welfare of the fish they sell. Thank God for not-for-profit forums like this that offer knowledge and experience.
Vern
PS I still have the 55 for breeding and the pleco is now keeping my S.American tank clean!