I can remember that it was nice to see community members backing up sellers offering something for a good price, or something that was particularly niche or hard to find. It’s all very well for a seller to say things like “one-of-a-kind”, “great price”, “super rare item that you need for X, Y, or Z situation”, but having community members comment with feedback like “I’ve had mine for five years and love it”, “I spent three months looking for one”, “good value – if it was my size I would buy it now”, really adds value to the ad, supports the seller, and gives potential buyers more confidence too. Perhaps this is open to abuse by sellers posting under multiple accounts, but those who frequent the forums will get to know who is trustworthy and who has certain specialist knowledge. There are a few Hubbers whose feedback on items I would take very seriously.
Also, while there might be a few selfless members sending feedback or suggestions directly to the sellers (thank you!), it adds a barrier that I think would deter most members who would otherwise quite readily post a quick clarification or question in the comments.