But there is a huge difference between "No returns" and "All returns, including buyer's remorse and precipitous drops in value."
I have always gone with a "Satisfaction guarantee," meaning that if I did anything - poor packing, missed something grading, missed a crack in the slab - that I would make it right. That could be return and refund or partial comp for the screw up. I am down with any and all of that. I don't know how I would feel about "Hey, I bought this 10 days ago, now there is one on Ebay for $50 less, I want to return it."