Thou shall confirm what the dealer means when he says he'll refund your money. Thou shall beware independent, third-party grading. Be especially careful if the dealer himself offers the certification or authentication. That's like asking the fox to guard the hen house. Thou shall know what grading and appraisal terms mean.
Gemstones can have various colors and flaws.
Thou shall comparison shop. Not only is comparison shopping the only true way to know if you're paying the best price, but it's also part of the fun of collecting. Comparison shopping means you avoid the sight-unseen market, and while buying certified or authenticated coins, stamps or autographs may give you some assurances, nothing beats checking out the item yourself. Always check industry price guides. Check ads in newspapers, check at stores, and even check at flea markets and garage sales for clues about the true worth of any collectible.