Wondering about forced tags

[SFGHQ]

I was wondering if there was a mod existing, or if one could be easily created that would force a user to choose from a list of tags, to tag his post. The tag would be added to the front of the post title.

For example, say this is a buy/sell forum. The user needs to pick whether he/she is buying or selling. Once they pick the title is edited to include the tag. IE:
[Buying] A new car. Or [Selling] An old car.

I think if users were able to create their own tags, this would be a very useful mod that can be used in hundreds of types of situations.

Any ideas?