I tried the Jeffrey's EXIF viewer with three images taken with the same camera (a Canon 50D) and three different lenses. The 'Internal Serial Number' value returned by Jeffrey's EXIF Viewer was the same value for all three images taken with the same camera and three different lenses. Each image was taken with, respectively, a Tamron SP AP 17-50mm f/2.8 XR Di-II, an EF 70-200mm f/2.8 L IS II, and an EF 28-135mm f/3.5-5.6 IS. The two Canon lenses were correctly identified in the "Lens Model" field (The Tamron was identified as a "Canon EF 28-70mm f/2.8L or Sigma or Tamron Lens" because many Tamron and Sigma lenses are known to spoof the lens ID for the older EF 28-70mm f/2.8L). This leads me to believe the "Internal Serial Number" value is a reference to the camera body, not the lens (at least on older Canon bodies that do not differentiate between two different copies of the same lens model for various purposes such as AFMA). Even though the the lens ID immediately precedes the "Internal Serial Number" value, it may not accurately reflect the serial number of the lens used to create the image. Images taken with my 7D using two different lenses shared the same "Internal Serial Number" with each other, a different number than the three images taken with three lenses using the 50D shared. Images taken with my 5D mark II have a blank "Internal Serial Number" value when using Jeffrey's EXIF Viewer.
Many Canon camera bodies have a separate internal serial number that doesn't match the one stamped on the exterior of the camera, at least not when the number in the EXIF is translated using a standard hex to decimal converter. This may also be the case regarding lens ID numbers with bodies that can differentiate between two different copies of the same model lens.
The value for 'Camera Serial Number' in the EXIF maker notes of images taken with my Canon 50D is 5AA411141. Using a standard converter yields a decimal value of 24331227457. Yet the Serial number stamped on the camera body is 1520708485. This number is correctly reported by Digital Photo Professional as the camera's serial number. Irfanview reports the serial number as "1520708485 (5AA411141)". Hmmm. The HEX number that correlates to the stamped serial number 1520708485 is "5AA42B85". The first four digits match, but the rest doesn't? This is very interesting, though, because (HEX) 2B85 = (Decimal) 11141! Thus it seems the internal number in the EXIF information is a combination of hex and decimal digits! A four digit hex number (in my case "5AA4") followed by the decimal equivalent of the rest of the hex form of the entire serial number (in my case 11141 which is the decimal equivalent of 2B85)!
The Value for my 7D and 5D II work exactly the same way. If I convert the camera's serial number to Hex, then convert the last four hex digits back to decimal, I get the same value that the maker notes show for the camera serial number: The first four digits of the 8-digit hex number followed by the decimal equivalent of the last four digits of the 8-digit hex number.
Although it doesn't help your situation, the best practice when selling a lens anywhere near the value of an EF 85mm f/1.2 L would be to document the serial number of the lens and the condition it is in before you ship it.