I MIGHT do it if I had both soft and hard water tanks that were under-stocked. First I'd talk to Jonny at Dan's Fish to see if he would let me know what fish were involved. He may or may not tell but we seem to get along OK at least to the point that he knows who I am.Whoah! I wouldn't do it!! Hahaha!
I don't disagree with what you are saying and I think Dan could handle it better (not addressing the hardwater thing but the grab bag); he has a few fish left over that he wants to dump them but of course no experience fish keeper should buy a random fish without any idea of species and that is the catch and why it is so inappropriate. If it is a tetra it is likely a species that should be in a group if it is a cichild it might have special requirements and yada yada yada it is just wrong and dan should know that but he is more interested in dumping the remaining stock that he can't hope to sell as a lonesome fish.I understand business desperation, but if I were in the US, Dan's would be off my list of sellers with that. I wouldn't buy from him after that.
Believing all fish handle all water is convenient thinking - it isn't analytical thinking with the welfare of the fish in mind. Believing a 'fish is just a fish", which would underly an idea like grab bag fish isn't what I'd expect from a dealer in rare fish. It's Walmart thinking. I won't be suggesting his place as one to check to American posters after seeing that.