I respect that you hold this opinion about preserving pricing information on the forum, but I would not personally recommend this approach to someone who is seriously attempting to arrive at a "market value."
Putting aside all the issues related to the condition of the instrument and the (very important) motivation of the seller, there is a bigger issue to consider when using the forum information in the way described:
As someone who has lurked this forum for quite a while, and who has been closely watching the marketplace sub-forum for at least the last three months, I can objectively say that there are enough listings here that are not what one might construe as "market value" as to be statistically significant, i.e. some listings are higher and some considerably lower than market. If this were a scientific study, taking the marketplace prices at face value would result in your "data" giving you unreliable results. In order to prevent that and improve reliability, you would already need to know enough about the likely valid values for instruments in the same or similar class as the one your looking to sell, and take into account current market conditions. Then you would have to be able to select the right sales information to discard from all of that information you've collected. But that type of knowledge isn't something you would pick up just by searching for past sales price in a post. And I personally would be concerned about taking a position that might offer someone the impression that, without knowing anything about an instrument for possible sale/purchase, a quick search that results in a thread shows a price is in any way reliable. Prices are sometimes useful information, but they are also unreliable.
Clearly, the price the item fetched, even if only being useful
sometimes, is more useful information to help determine an estimate than anything you will get from a post that has been edited after sale so as to exclude the prices.
Unless, of course, you’re new to the site, in which case, it might actually help you decide you’d rather not buy from sellers that revisit their posts to delete the selling prices after the sale.
That’s been a factor for me at least once when I considered buying a ukulele I really wanted. But I knew the seller had bought it here, a few months before, and the original listing was edited. I suspected that the original sale had included a very nice case, which was no longer included, though the price seemed not to have changed. Or so I thought. And though I could not verify this, I decided to pass on it, in large part because it all seemed needlessly sneaky.
So, as a newcomer, it would make me think twice about buying a ukulele here that I found to have been sold here, recently, to the latest seller, and I had no way to determine the price they paid.
But, by all means... delete your selling prices if you want. It’s not my site. But I do know sellers here that I will no longer deal with because they have misrepresented their items, posted photos that looked extensive but actually hid blemishes behind glare on the finish, and then edited out all the info and pictures of the item after I paid for it but before I received it.
In my opinion, it’s creepy and uncalled for in a market that is, or should be, doing whatever it can to foster trust.