That'd be a disaster.
"This gun is over priced"
"I'll give you half the asking price"
"I've always wanted one of these, but I can't afford it right now"
The Exchange isn't a discussion part of the forum, it's where business is conducted. The EE would just get way too cluttered if people could reply to postings. Not to mention certain items would bump others off the first page unfairly. The system isn't perfect, but it does work very well, as it is. If you have a question for an EE poster, then PM them, if they are interested in selling the item, they'll reply. If you wish to discuss a certain firearm, that's what the discussion forums are for.