Most "black market" guns in America are purchased legally across state lines

The data you posted doesn’t link through to anything regarding why certain outlets were chosen. It just shows general patterns on ownership nationally. When a store is turning up at rates double what any other store in the data does and ten times what other stores in its immediate vicinity does, then yes it is reasonable to assume that actual substantive difference would be made by pulling their license. For another analogy consider food processing. US standards are lax broadly, but we know that some producers are particularly egregious. Clamping down on the most outrageous offenders isn’t enough to fix the system, but it stops the additional harm they do. That’s exactly why there has been a huge push for clear tracking. As far as the idea that taking steps props up the broken system, I’ve generally found that not to be the case. When a society starts taking steps to solve a problem they tend to be willing to actually take steps to solve it more broadly.