LES is also correct.
I don't know of any state laws that prohibit bullpups by design even in the most restrictive states, nor restrict any other type of weapon to only active or retired personnel.
Matter of fact, Mossberg even sold a bullpup 500 for a while, even though it was ugly as sin and not a very popular model and several aftermarket stock makers even have various bullpup stocks from everything from hipoint carbines to old SKS bullpup stocks, etc.
It may be their store policy, but there certainly are not any federal laws written as such and I am sure that Mississippi has no state laws like that either because like my state, are very gun friendly.
I can give you a link to the state and federal firearm law books at the atf website if you want to reference them for your specific state, but there is really nothing prohibiting the dealer from selling it to anyone that can own any other firearm so approach that salesmen on your guard.
Except maybe by contract with the maker/importer (very doubtful) or that he just doesn't want to sell it to them as store policy, but don't let them tell you it's the law because if he does, at that point, it's time to do an about face and never go back.
And if you want to be difficult with him, ask him to show you the law he's talking about in the books.