This is going to be a long press release so I will post the link to "Mayors Against Illegal Guns" press release and only post the interesting points.
The Voluntary Code - A Set of Retail Best Practices - Goes Beyond What is Required by Law
Videotaping the Point of Sale for All Firearms Transactions. Participating retailers will videotape the point-of-sale of all firearms transactions and maintain videos for 6 months to deter illegal purchases and monitor employees.
Computerized Crime Gun Trace Log and Alert System. Mayors Against Illegal Guns will develop a computerized system that participating retailers will implement over time to log crime gun traces relating to the retailer. Once the program is in place, if a customer who has a prior trace at that retailer attempts to purchase a firearm, the sale will be electronically flagged. The retailer would have discretion to proceed with the sale or stop the sale.
Now lets look at Florida Law, this may apply to other States where registration is Illegal, but I am in Florida.
790.335 Prohibition of registration of firearms; electronic records.—
Statutes & Constitution :View Statutes : Online Sunshine
Although as Mayors they have limited immunity in their official capacity, MAIG is not an official Govt. Entity. Bloomy has NO immunity for his actions in regards to them. Has anyone in a state where registration is illegal subpoenaed his gun registration records to see if anyone from a state where registration is illegal, or anyone in general sued to find if MAIG has them on file as a legal gun owner. In Florida this could be a class action suit to see their database with both Criminal and Civil redress. Anyone from Florida on the list could retire with the money from Bloomberg. I think there are other states where registration is illegal too but I do not know them right off.4. (f) 3. Except as required by federal law, any firearm records kept pursuant to chapter 538 or chapter 539 shall not, at any time, be electronically transferred to any public or private entity, agency, business, or enterprise, nor shall any such records be copied or transferred for purposes of accumulation of such records into lists, registries, or databases.