The thing is, if you leave the original asking price up there and also mark it SOLD, when you actually accepted less, aren't you putting erroneous/false information out there? You are creating the impression it sold at the asking price you left up there. Maybe there is a middle ground of just marking it "sold for less" or "best offer accepted."
Sold for $$$ amount is a much better idea if your goal is to help the community by building a baseline of prices paid.
Listed for not so much. After all what if I list a Sebenza S35VN for $2,500 but actually sell it for $300-$400? Of course this is exaggerated but point stands valid.