for example, Pterygoplichthys spp have more then 10 rays on their dorsal fin.
but in general, sellers just forget the names given to them by breeders or exporters. sometimes fish comes in without a scientific name, so they just label them as being plecostomus or something like that.
buyers must get more responsible. do your research before purchasing a fish. take a picture at your LFS and ask around if you're not sure which species you're dealing with. a forum like PlanetCatfish is of great value in this proces*
* and ofcourse for a 1000 more reasons!
![Cool 8)](./images/smilies/icon_cool.gif)