Many aurei for sale these days are modern reproductions, which is the only way most collectors will ever have an ancient Roman aureus coin in their collection. This is certainly a valid way to do it, since authentic aurei are very rare and generally reserved for the most deep-pocketed individuals and organizations. If you are looking for a replica aureus, note that such copies should be clearly marked, in the listing and sometimes on the coin itself.
But yes, there are also a few actual authentic aureus coins for sale, and if you are in the market for one it is worth watching online sales listings.