Trader rating is somewhat self explanatory, but kind of confusing at the same time. Basically its a way to say if a person is a good buyer or seller based on the unofficial transactions that go on at i-club. Its not tied to any specific transaction (like eBay's feedback system), so it can be abused. The complexities really arise in how its used. Because it can be abused no one really pays attention, and because it can be abused, not many people use it. On the other hand, people are generally not as likely to sell/buy something from someone with a negative rating. So, this means if someone else decided to be mean, they might be able to destroy your ability to buy/sell things on i-club (and since its not tied to anything, anyone can give anyone else one negative point for no reason).
Scooby snacks are a little more interesting. If you make a post in a technical forum on i-club, you can get anywhere from 1 to 3 scooby snacks depending on how 'technical' the forum is considered. Like, in the general forum, or motorsports forum, you get one scooby snack for each post. In the engine management, suspension, tires, brakes, etc. forums, you get 3.
You can then use these scooby snacks to 'buy' things on the board, like avatars, custom titles, italliacs for your username, glowing usernames, etc.
more info on what you can buy here
http://www.i-club.com/forums/store.php?s=&action=main