Inflation... Deflation... Short Sales?!?

Grinding into the night we are currently thinking about our database schema and just in general how markets work... None of us on this team would say they are an expert trader, but with the help of some google searches we are having pretty meaningful design discussions about topics like market inflation/deflation, if we should or can allow short positions, do we allow users to have margin trading capabilities, etc.

Of course we are keeping in mind that our market holds no real world value, but we do want it to behave and react properly, and even more importantly, we want it to do what people would expect.  But even along those lines, some hard questions arise:

• In a game design that calls for lots of free money being injected into the market all the time (new users, daily log on gifts, incentives for getting friends to play) how do you combat massive inflation?  
• Can we even allow short sales?  What if a user can't cover a short and goes negative?  
• How can we possibly store every transaction ever made and not have our database host want to murder us?
• How do we exercise market control as invisibly as possible, or at the very least without making a user ever feel cheated?

It's a fun topic, and has resulted in many, MANY tweaks being made to our database schema.  That's why you keep it on a whiteboard.