They need a price patrol.
The problem with your theory is you'd only want it applied to items you think are overpriced.
Let's say you post a Glock 19 for $500. Then I come along and review your ad, but I think a used Glock can't possibly be worth more than $400, so I tell you your ad will need to be updated to get your price in line. Are you then going to sell your item for $100 less that you think it's worth? The entire concept is beyond ridiculous.
The rule here has always been that someone has the right to list an item they own for whatever price they want to sell it for. That rule is not going to change. Period.
There is a very simple procedure to handle items you think are overpriced:
Step 1: Hit the back button on your browser.
Step 2: Move on the next ad.
Repeat as needed.
If an item is in fact overpriced, it won't sell. If it's simply overpriced for you, it just won't sell to you. Either way, problem solved.