Hi ccc & welcome to the forum.
First step would be to check the last MOT online & see if it was listed as an advisory item. If so I would be pushing for a replacement belt as part of the purchase.
Seat belt condition can be a divisive subject amongst testers. There are two sanctions, Major fail for a damaged belt, and Dangerous for webbing significantly stretched or weakened.
I know some would be inclined to fail, others to pass & advise. I’ve also come across those who would take a lighter to the frayed edges to seal the fibres.
My own opinion would be a Major fail as it’s not just a bit of fraying along the edge, the fabric of the belt is seperating & deteriorated.
There may be others who disagree of course.
