It's on you to make sure the item you sell is legal in the buyer's state. GunBroker doesn't get involved in that AFAIK. IME, GB doesn't get involved in much of anything, but that's another thread.
It's less a problem with firearms that have to go through an FFL than it is with states that make, say, mags with more than 10 round capacity, illegal to sell.
The buyer wants the mags, you want to sell them and, if you sell them to someone in one of those draconian-lawed states, you MIGHT find yourself in trouble.
Most FFLs, OTOH, know what is legal in their state and won't get involved in some problem-purchase.
Lots of ads on GB and AA say "No sales to CA, NY, NJ, MA, etc. even when the item might be legal there just because they don't want to risk it.
Bob