Trader grades reflect how responsive a collector is to trade offers. Having an A+ grade does not mean this collector is more likely to accept than decline an offer, but it does mean they will respond. Traders receive lower grades for allowing trades to expire without responding.
All collectors start out with a trader grade of B, and depending on how responsive they are to trade offers they receive, the trader grade will improve or worsen. In this case, Happy the Dog has a trader grade of A+ (the best possible).
You will also be shown collectors' trader grades when you see a list of owners of artworks. This helps you determine who you may want to propose trade offers to.