Thanks for your answers, everyone. Just to clarify:

There is no law prohibiting me from driving to the dealer in Virginia with my legally-owned pistol. Maryland law does require it be unloaded, with the ammo (if any) packaged separately, and carried in the trunk of the car. I don't know if there are any municipal laws that apply, other than those in Washington, D.C....but while I will be driving around Washington on the infamous Capital Beltway, I will not actually be driving in the city.

As LoboGunLeather correctly surmised, I am interested in doing this because it would be much cheaper to drive to the dealer in Virginia than to ship it there. The buyer is a LEO, who deals with the FFL regularly, so I suspect there will be no problem with them accepting the pistol from me to transfer it to the seller.

Yes, I am leery of running afoul of the ATF, which is why I started this thread. Thanks again for your responses!