I just thought of another tip/trick that could be added;
My mouse tends to double click on accident. How do I avoid sending multiple trades/posts/etc?
When you're done typing your post/trade message, press the Tab key once and hit Enter. If you're making a post via the Full Editor (Not Quick Reply), or if you're PM'ing someone, you'll need to press Tab twice. If you're sending a trade without a message, you should at least click inside the message box before pressing Tab.