Sold two to a member years ago that they were investigating for international gun running. They wanted to know if I had the serial numbers. Told them I didn't and they said thank you and goodbye. No billow sale either.
Had something similar years ago. ATF called and were investigating someone for international gun running that I had sold two guns to about a year apart. Told me I had done nothing wrong, but wanted to know if I had the serial numbers for the guns. Didn't have them and I never heard anything else.