While ultimately the best advice is likely go buy from a true expert that is trustworthy. Unfortunately that not always the case.
For what ever it’s worth, here are few pointers I would Highly recommend.
Buy only a GIA or AGS certified diamond.
Buy a diamond with complimentary grades (Avoid contrasts such as high colour with low clarity or excellent colour & clarity with lower cut).
Buy only natural untreated diamonds.
Florescence grade can have a great impact on a diamond visual & price. “None” is the premium grade, with great tolerance for faint. Medium Blue works well with lower colours such as I & J As well as None and Faint. Resist the discount and avoid Strong & Very Strong grades.
When possible and for sure when at par, choose a Canadian diamond (With the maybe only exception for Argyle Pink).
Diamonds have a great social function when getting engaged or adorning a loved one with jewellery. Buying a diamond as an investment is a more involved proposition and require a higher skills and expertise.
These are very basic but important pointers IMO. I still highly recommend working with a trustworthy expert.