I'm not entirely sure you're correct on that one.
For one: there is no way to "deregister" a regulated firearm from an owner in the MD system, ever. Once it's registered to you, it is registered to you FOREVER. Now, if you sell them to someone in MD, they would *theoretically* be able to see that they were registered to someone else after you, assuming the database isn't as messed up as gun databases usually are (look at the NFARTR for an example). However, if you sell it to someone else out of MD, the MD system will have it registered to you FOREVER. I have sold several regulated firearms to people in other states, and shipped them directly to an FFL in that state. As far as Maryland is concerned, those guns are still registered to me. If MSP ever showed up at my house to seize my guns, all I could say would be "sorry, I sold that years ago to someone in Florida."
Thanks for the clarification. Documentation of having sold a particular weapon could possibly preclude a LEO from having probable cause to arrest you for violating the ERPO and not turning the gun(s) over. But he/she might not be interested in being a finder of fact at your doorstep. As far as the LEO is concerned, he/she is dealing with a judge/commissioner's order to go get your gun(s), based on someone's sworn Complaint that you have them, or take you into custody if you don't comply.
All of this goes to the lack of due process which this law presents. Unless you luck out with a sympathetic LEO who leaves without arresting you, you could still get arrested and be forced to defend the ERPO, even if you in fact previously sold a weapon.