Watch real trades, not just asking prices
A seller can ask any amount. What matters more is the price at which players are actually willing to complete trades.
Prefer liquid items
An item can look valuable but still be difficult to resell. Demand can matter more than a theoretical top price.
Leave room for profit
A better entry price gives you more flexibility and reduces the chance that one bad trade wipes out several good ones.
Avoid emotional trades
Do not increase your offer just because another player is pressuring you. The ability to walk away is an advantage.
Keep simple records
Track what you paid, what you received and your resale value. Over time you will learn what actually makes you Diamonds.
More resources
Read the Trade Server guide or compare Diamond packages.