"Correct price" is not a boolean field. The idea that there is a universally "correct price" is a very condescending and patronizing idea.
1. There are a number of items for which it is really difficult to gauge used prices.
2. The keenness of the seller, the urgency for the buyer, the current utility and rarity of the item, all affect the price and may not be known to price advisors on the thread.
3. Stating a price in the thread creates an anchoring effect - people who previously were happy doing a deal at price X will now be unhappy just because of a comment that the article should be priced at Y.
4. Price comments are also harmful to market health as it leaves a bad taste in the seller's mouth. People might not prefer to sell on forums where uninvited non-buyers get into arguments with them.
The correct price for a transaction is what both parties agree upon, and buyers can negotiate for themselves if they feel like. If a buyer wants to buy they will buy, if not they will move on, the item will remain unsold, and the seller will reevaluate or hold on depending on their situation. This is a self-correcting situation.
People who comment on prices in a thread without any intention to purchase think they are doing a great social service but in reality they are just smart alec busybodies.