Personally, I wouldn't do that. The reality is, you have a darn good idea of who it is so what point are you trying to prove? Brokers exist for several reasons, one being anonymity. There's no point in putting up a wall to negotiating, what are you actually gaining by doing that? Just set your price and stick to it, if the buyer isn't who you thought it was, who really cares, you got your price anyways. Just price it high, stand firm and let the chips fall where they may.
Another reason you don't want to do that is, you don't want to give them any ammunition legally. The CIRA dispute rules say that you cannot register a domain simply to prevent access to the domain to another business. You want to establish that the price is the same, regardless of who the buyer is, not give them the impression that your pricing is jacked up just for a certain buyer.
What you can tell them is that if the seller doesn't disclose their identity and billing address, then you have to assume the worst possible scenario and thus you have to collect an additional 13% HST to cover your ass if CRA says you needed to collect it after the fact. CRA can assess you for the taxes if you didn't collect it when you should have. So tell them if they choose not to disclose, then you'll further be required to add 13% HST on top of the asking price.
In fact, you _could_ follow up saying that the reason you need to know the identity and billing address of the buyer is to ensure you quote a price that includes the proper taxes. That at least covers your ass as to why you needed to know their identity.
Best of luck.
Rob