My Experience with KEH
I have been buying from KEH for years, and yes they are very fair and conservative with their ratings.
I decided to send them a Pentax Spotmeter V a few years back. I had purchased it used several years ago. The gentleman at KEH received it and called me with a fair offer, which I accepted. He then called back about two days later and advised that the meter was one internally modified by Zone VI (it had no external Zone VI markings noting this).
He offered me a much higher amount.
That was service!!