If you want species ,I would say that there will be less of a chance of mixed up or wrongly tagged with reputable orchid nurseries.
In the event of mixed up, the nursery may be able to trace back and correct the mislabel.
Unless the private seller still has the old tag from the original nursery, and he/she is a very knowledgeable orchidist, and is very careful in keeping his/her plants' record.
How would we know for sure the person with a no-name plant would not look up flower pictures and assign whatever name seems most matching?
Recently, I was looking for den. nobile species, and see one on ebay, the flower is attractive, looks close to the species flower norm, and the price is right.
However, when i ask whether the seller still has the old tag or record of which nursery he/she bought from. The answer is negative. I am really not sure what to do now that there is no record.
Actually it is worse in dendrobium nobile's case, since this species has been used in breeding extensively and because a lot of sellers or even nurseries don't even bother to call them nobile hybrids so.. and .. so .
They just call them nobile.