Hi Paul,
Please don't take this as me attempting to scupper your sale - quite the opposite.
Just to make you aware, there is currently a large number of counterfeit Takata belts on the market which may be why nobody appears to be biting on this.
To put a potential purchaser at ease, I recommend posting a picture of the FIA homologation label and hologram to demonstrate that this is a genuine product. I've attached a sample image below.
Since 2013, the FIA has mandated that many safety devices (belts, race suits, helmets, fuel bladders, HANS etc) must carry an FIA hologram. This is to fight counterfeiting and to ensure manufacturers consistently produce quality products to protect consumers.
Statement regarding counterfeit belts on the Takata website here: https://www.takataracing.com/landing.php
Good luck with the sale!