So, I know this is an old thread, but anyone coming across it may want to know the actual federal law.
18 USC § 922(b)(1)) states:
(b) It shall be unlawful for any licensed importer, licensed manufacturer, licensed dealer, or licensed collector to sell or deliver—
(1) any firearm or...
I think this guy definitely needs to fight the HOA, maybe advertising is defined in the contract, but I'm betting it was intended to keep signs out of yards and off of houses, such as "new roof or windows by Joe Schmoe." Now if he had a Marine recruiting poster hanging on his garage door maybe...