It's a noble enough idea, IdeaMan, but it won't work.  I don't think I can explain it better than others have already tried.  So how about we play a game?  We can pen & paper a small bitcoin economy with an exchange.  You play the bot, I play a forex trader, someone else plays the rest of the market which is trying to buy M&Ms for a price that's roughly equivalent to USD$2/pack.  I'll start with 100 RevCoins, you'll start with $100 USD.  You try to hold a floor of $1/RC.  We take turns issuing market orders.

In the end, I guarantee that:
You can prevent the price from going under $1, as long as you don't try to do anything else.
I can make the price fluctuate wildly (several orders of magnitude).
I will end up with all your USD.
You probably won't even end up holding all the RevCoins unless you stick to a straight $1/RC exchange rate.
Some M&M buyers will get screwed for a notable percentage of their money.

I'm not sure I actually am up to really doing this (it'd involve a few hundred trades to show you all the ways the market will slap you around and achieve all my goals above), but I'm hoping just thinking about how you'd approach it will make you think about where the problems are.  It's an exercise you should certainly try doing on your own notepad, though.