Could we have a shut up rule?

Could we have a rule that enforces what people can say in certain forums? This would mainly apply to the market forums, and guide and graphics sales. It's really annoying to see people post on a thread, when they show no interest in buying the item being sold, but just want to spam.

This would get rid of things like:

"I don't have the money for that, but good luck selling!"

"Omg what a scammer don't buy from him"

"Why would anyone buy this?!"

It's annoying when people are trying to sell something, and others are making the item being sold look less valuable etc.

So, a rule where people can only post when they are willing to buy something, or are inquiring about the item being sold.

Not sure if it's been suggested before, but this search feature is horrible. >.<

Thanks for reading.