Actually, people do do business on the "I'll take it" system all the time. In situations where someone wants to 'hold' the sale, a deposit is offered in return. Even still, the vast majority of items are sold on a first come basis. If I have one Corvette to sell, I'm selling it to the first person that hands me money. Claiming you'll 'think about it' doesn't do anything for anybody except the person that's 'thinking'. This rule is just about as basic as it gets; if you love it, put a ring on it.
These rules have been developed as the forum has grown. None of the rules you'll find here were created on a whim. In fact, the moderators as a whole, do not like adding rules. They are added when needed to ensure that this forum remains different (read: better) than others.