Firearms are capable of being used for legal purposes, as they are in the overwhelming number of cases. A radio that transmits on a restricted frequency is illegal. The ruling doesn't say legal frequency transmitters can't be sold, just that the broker (ebay) is responsible for ensuring it's not facilitating the sale of illegal radio equipment. Ebay is the one choosing not to sell most radio equipment (even if legal), because they don't want to deal with it.
Firearms used to be widely available at big-box stores, as they were legal (and still are), provided the store prevented illegal purchases. Most box stores/locations no longer sell firearms, because they don't want to deal with it.
Longer answer would be that there are already systems in place to prevent transfers of firearms to and from parties that should not possess them in the US. Purchasing a gun at a sporting goods store involves filling out paperwork that can take a long time.
Buying a ham radio is as simple as going to a store that sells them and giving your call sign.
You could (depending on where you live) shoot off a gun in your backyard as long as it doesn't disrupt your neighbors. You can't key up a radio in your backyard on an band you don't have a license for without potentially disrupting someone in some other part of the world.
https://en.wikipedia.org/wiki/List_of_amendments_to_the_Cons...