First of all, from reading the auction it appears to me as if it's a custom piece made by the seller himself.
Second, there are many ways to tell if a Diaclone Swoop is real or not. First of all, there are many mold changes between the Diaclone and TF versions. The beak on the Diaclone is sharper (both at the tip and the standing up piece above the eyes), the tail is much sharper, the metal bar between the knees is not there (why they ever did that for the TF version is a mystery to me), the feet are different, and the Diaclone version doesn't have plastic rings around the screws that make the chest slide in and out.
Also, the chrome on the Diaclone version is indeed gold, not silver.
Here's a pic for reference:
And it should be noted that the quality on the knock-off Swoops is not nearly as good as on the originals, be they TF or Diaclone. The knock-offs don't even stand up straight (the one in the auction is leaned against a white wall), and the launcher falls out of the peg on his wing. You have been warned