Agree with the above suggestions.
FWIW, I'd also be sure it has a Platinum Report from AGS in New York.
If it doesn't have one already I'd call AGS to get one.
I believe that that to be the most reputable document to justify the appropriate price for these very rare and fine gems.
As a buyer AGS Platinum is the only document that would get me to open my wallet for what claims to be a Kashmir sapphire.
This is true even if I'm buying it from the most reputable jeweler on the planet as vendor self-grading carries an inherent conflict of interest, but the AGS Platinum Report is especially important if I'm buying from a private party.
Then, to ensure the stone matches the report, the gem would have to be sent to an independent appraiser, such as one on Pricescope's list: https://www.pricescope.com/appraisers?searchby=name&searchvalue=
Then that appraiser would act as an escrow service who sends the funds and the stone after verifying everything is copacetic.