Its called incrementalism. They take your rights one little bit at a time. They will get away with whatever they believe they can. The administration believes there is finally "consensus" on "common sense gun safety" measures. They will get away with it and the only hope is that legal challenges reverse the overstep. Only problem is, it only takes the cost of the ink to sign something like this, but it will teams of lawyers lots and lots of time to try to make it go away.
They will start with mentally ill people because "everyone can agree" that this makes sense - Right!? You can't have some maniac running around with guns..............
As for the Sales end of the EO - I don't know how they would presume to enforce that. Would they post sentries at all the gun ranges to check serial numbers and request proper registration papers? If your buying ammunition, will you have to supply the information for the gun that it will be used in? How far will they go?
Remember the excuse is "If we can save just one life - it will be worth it".