All legal issues aside...having worked and lived in Egypt for a period of time, I can tell you that Egyptian antique market vendors are merciless when it comes to foreigners. They know you have the money, their job is to extract as much of it as possible from you. Be prepared to drink lots of tea and bargain hard (but courteously).
You won't find good deals on real antiques of any sort in the big markets in Cairo, it's best to get a local to help you find a non-tourist shop. Of course, by bringing you to a dealer he gets a cut that's factored into the final price, just like the restaurant and hotel touts...
If you do make a purchase, get in a cab and back to your hotel as quickly as possible, so the police (or people posing as police) don't get wind of it and hit you up for a bribe.
Bottom line, unless you speak Arabic or have a trusted local friend with you it will be difficult to get a really good deal. I wouldn't try it myself unless I was quite confident in my knowledge of antique firearms (which I'm not).