The best method is to get a price from 2-4 gold sellers, then use them to your advantage, saying well x user has offered me this price, then they'll ask the other user and they'll agree, so then they offer you a better one etc etc. Depending on how much gold you're buying/selling, and the demand for cash/gold, you can get really good rates. Some sellers are stubborn however and won't bother with negotiations but that's how the market works.