Your stock market idea is interesting, however it wouldn't work well at all and would destroy the entire pro scene. If you check http://dotabuff.com/heroes/played you'll see the list of most played heroes and their win rates. The heroes who most likely need buffs will be at the bottom of that list. In a stock market simulation you would have people bidding on their favorite (popular) hero or against their most hated (popular) hero. Heroes like Invoker who have a 40% pub win rate are actually balanced for pro players with a 50% win rate. Given the popularity of invoker and the low win rate, we could expect a good amount of buffs causing the pro-metagame to shift to invoker being #1 pick.

Basically, balancing is hard.