The Problem of Over-Grading
When selling polished goods, it is important not to insult the dealer-buyer by over-grading your merchandise. It will make you look dishonest and unprofessional. If you're selling to an unwary private party, he may easily get it checked by an independent source. If the represented grade is inaccurate, the buyer will be disgusted by your dishonesty, asking for a refund. The knowledgeable dealer or gemologist who checked the stone, will also be aware of your practices, and will likely spread the word in the industry, probably hurting your reputation.