Check before you accept
Run every non-trivial trade through the analyzer. It takes about fifteen seconds and it is the whole reason this site exists.
Prefer items you understand
Thin, high-value items are much harder to sell at the number on screen. For your first few trades, favour items with strong demand and a long sales history.
Watch for untradeables
Limiteds that were disabled, delisted or flagged for resale limits cannot be traded normally. Check the item page before you build a plan around something.
Never trade items you cannot replace
If losing one copy would genuinely hurt, do not put it in the trade. This is the fastest way to end up someone else's warning story.
Keep records
Note what you traded and roughly when. If you are building a collection, knowing what you paid versus today's value is the only way to know how you are doing.
Ignore hype
Group chat activity and social trends move ahead of real value, not behind it. If everyone knows about an item, the price is usually already in it.
Put it into practice
Run your own numbers before you accept anything.
Open the Trade Analyzer Browse values