Do I need to use an FFL to sell them to someone online?
Now that you mention it, I’m not sure if gun magazines are legal to sell. I know that in some states, like California, there are laws prohibiting the sale of high-capacity magazines. So it would depend on the state you’re in.
Another thing to consider is what kind of gun magazine you’re talking about. For example, AR-15 magazines typically hold 30 rounds, while AK-47 magazines can hold up to 60 rounds. There are also drum magazines that can hold even more rounds. So the type of magazine would also be a factor in its legality.
Generally speaking, though, it seems like selling gun magazines might be regulated more than simply selling guns themselves. So you would need to do some research on the laws in your state before attempting to sell any magazines.