Apparently, this is the Maryland way.
There are essentially two legal ways to sell a firearm in a private sale in any state:
1. Strictly in accordance with state and federal laws.
2. In accordance with state and federal laws AND whatever additional stipulations one may add on top of that.
Notice the common denominator between the two.
People will quite often have a cow over those "additional terms", but in the end the seller can dictate whatever terms he wishes and it's up to any potential buyer as to whether those conditions are satisfactory for the deal. If not...then there's no deal. Personally (and professionally), all terms of any potential transaction are to be clearly established up front such that when the sale actually goes down, it's strictly in accordance with the terms at the time the deal was made. So if a copy of a driver's license was not a condition of the deal, the seller cannot then demand this at the time of the exchange; nor can the seller renege and not provide this if it was already established as a condition of the deal.
I really don't care what conditions the seller has, so long as everything is legal. If the conditions are not acceptable, and non-negotiable, then I'll pass it up just as I would if I didn't find the selling price acceptable. No big deal. And no harm in asking up front, either, because that's the nature of negotiation. But there should be no animosity on how a seller chooses to sell his own property.
Which means if I set the terms of the deal as the buyer showing up in clown shoes, dressed in a school girl outfit, 5 days of beard growth, a patch over one eye, and smoking two cigars (one out of each nostril) and the potential buyer sets the deal under those conditions, then he/she better darn well better show up in clown shoes, dressed in a school girl outfit, 5 days of beard growth, a patch over one eye, and smoking two cigars (one out of each nostril).
If the buyer can't meet those terms (like can't smoke because of emphysema, for example), then the buyer can ask to negotiate a different set of terms, such as a party blower noise maker in each nostril. If that's acceptable to me, and we set the deal, then I would expect the buyer to show up accordingly.