Trader grades reflect how likely a trader is to allow a trade to expire. Having an A+ grade does not mean this collector is more likely to accept a trade offer, but it does mean they will respond one way or another. Traders receive lower grades for allowing trades to expire without responding.
All collectors start out with a trader grade of B. Depending on how responsive they are to trade offers, their trader grade will improve or worsen. In this case, Happy the Dog has a trader grade of A+ (the best possible).
Trader grades are a helpful way to choose who to trade with. They are shown when you view a list of owner of a card.
Trader grades can be improved over time. When you propose a trade, your trader grade improves. Likewise, when you respond quickly to trades offered to you, your grade improves.