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davidcyclebackBarry is correct. A lot of sales are likely negotiated via email in response to a posting.
Personally, I wouldn't leave up a sales price. Neither as a seller or a buyer am I interested in others knowing what was paid. I don't desire my name and expenditures/sales to be recorded for public service. I know of others who don't want telegraphed what they paid for items.
If someone with a notebook keeps track, the postings are of course fair game and posted on a pubic forum.