To borrow a line from Bobby McFerrin- Don't worry, be happy.
IF you are selling a gun to someone in another state, you will be shipping it to a FFL in their home state. IF the person may not possess that gun in that state, their FFL will not complete the transfer.
However, the buyer CAN be a FFL, that will resell the gun- in another state. Have a friend that is a CA 01 FFL- he buys guns that cannot be sold in CA- but he is selling them (reselling) to another state. As a dealer, he can do that.
I WOULD include wording in my auction to the effect of "It is the buyer's responsibility to determine if the laws of your state will permit you to own this firearm. If in doubt, please do not bid."
You can also look at the restrictions from other gun sellers- and follow suit.
What we have here is... failure- to communicate.