Just for clarification, it's my understanding the BATF regulation requires the s/n be somewhere on the frame . . . not necessarily on the butt.

Federal regulations at 27 CFR 478.92(a)(1) add that “a licensed manufacturer … must legibly identify each firearm manufactured … [b]y engraving, casting, stamping (impressing), or otherwise conspicuously placing … an individual serial number” on the frame or receiver, and certain additional information - the model (if designated), caliber/gauge, manufacturer’s name, and place of origin on the frame, receiver, or barrel. Regulations further require the serial number to be at a minimum depth and print size, and the
additional information to be at a minimum depth. Additionally, the serial number must be placed in a manner not susceptible of being readily obliterated, altered, or removed, and not duplicate any serial number placed by that manufacturer on any other firearm.

Some models (like the Regulation Police) have the s/n stamped on the front grip strap rather than the butt. The S&W factory has been known to re-stamp the s/n on the left side of the grip frame when drilling through the butt s/n to install a lanyard ring on a finished firearm. And I would argue that a legible s/n in the yoke cutout of the frame would also qualify as being "on the frame or receiver".

Unfortunately in this case the legible s/n on the butt is obviously incorrect . . . the s/n in the yoke cutout of the frame has been obliterated . . . what may have been a s/n on the left side of the grip frame has also been obliterated . . . and the s/n under the extractor (even if correct) doesn't count for BATF purposes since it's not "on the frame".

I do agree returning it to the seller is step #1. If the seller is a licensed dealer and balks at accepting the return, let him/her know your next step is to contact BATF and report your findings as well as the source of the firearm.

Good luck,

Russ