The GM is right. It is listed as an extinct animal not as an illegal to own animal.
If you are gonna buy one, I would definately get a GM to sign off that it is a authentic silver steed. That way if there is a problem, you will at least have that GM's response on file to fight the actions.
You shouldn't have a huge problem getting them to authenticate the silver steed, most good auction houses run across questionable items all the time and they are authenticated by GMs.
This is not to say that there won't be people to page about it though......