Jewellery is one of the most highly regulated sectors of the luxury industry. What you're told will need to be put in writing on any receipt. Melee won't be GIA graded with each diamond bearing a report, but each parcel of diamonds are colour and quality graded, what he/she says will match the receipt (hopefully). Tell him/her what you're looking for, and also what you're not. If 10ct is not acceptable, but 14ct is then just say so. Yg is easier because usually the yellower it is the higher the carat, double-check rg and wg. Also, no setter in their right mind will put the best stones in low grade gold (or silver). The best stones are saved for high carat gold. These are rules of thumb, but check by preparing. Also ask about policies on guarantees for issues or repairs before he/she closes the deal. And take your time, ask plenty of questions. Maybe think over any purchase overnight and then return (I hate pressured sales).
Buy and bring a 10 x loupe with you, if need be, bring some diamond-set jewellery you know is good to compare especially for colour, clarity fire/scintillation/brilliance too. Precious metals should be hallmarked for min standards, know what symbols you're looking for when you check (each country will be different). Your local jeweller won't want to be put out of business, and he/she will want your repeat business so it's more a case if you'll get on. There are horror stories, like in all trades, but usually most jewellers are people who just want to make a living.
That you had problems at Tiffany, just shows you, no jewellers is immune and we have to think in terms of buyer beware and do due diligence wherever.