There would have to be a lot of people involved in the scam to be able to give you negative or positive rep as and when. The same person can't give you negative rep for one post and then negative rep for another straight after. It's like if I give positive rep to Famine for example (happens a lot) I can't give him positive rep again until a certain time, or ammount of rep has been given to other members, with the aim of fixing it to give somone loads of positive or negative rep, is probbly far too time consuming and awkward for anyone to bother. I've found, 90% of the time the system balances out, like getting positive rep for a nothing post, then getting no rep for a really helpful one.
I thinkt he ultimate goal of the rep system is for certain members to be noticed for good quality posts, the ammount of rep you need to get there is quite high, and not counting the mods theres only three or four members that have it. If you don't have the quality post badge, it doesn't matter how much rep you have. Like I said, it's take so much time and effort to properly scam the system, it's not really worth it, and if someone is found to be attempting to scam it, the mod's take action.