If all data one has is from reported sales, then that’s all the “data” one may obtain and then report with any degree of factuality. “According to reported sales data, these are the numbers reported”, etc.
Since unreported sales are just that, unreported, any estimate of unreported sales numbers will always be nothing more than a guess no matter who is doing the reporting. Are the numbers of unreported sales making the true numbers of firearms transactions greater? I will say most likely, almost definitely, yes.
I’m not vouching for the veracity of any news outlet, I just added this since it was new and shows the trend in numbers that are most easily verifiable by checking data compiled by the FBI.
Stay safe.