My opinion on this varies somewhat. First, I would never research values on a smallish community site like this (could never provide enough examples and varying conditions of specific parts). There is much more volume and variety on a site like the samba / ebay. I often edit the asking price on my ads once parts have sold (and that's all it is is asking price....you don't actually see what anything has sold for) as a courtesy to the buyer. If they wish to resell an item I don't want them to be restricted by what the item previously sold for (values change and sometime items are undervalued to move them).
When I sell something..I usually edit the title to say sold and edit my last post to say sold. I don't see the value in hitting reply to say sold as it bumps the ad up and clutters up the classifieds for people still trying to sell their stuff.
I'm all for leaving the rest of the body of an ad up. No need to waste any of the admins time. There are times I wish I could delete my own ads though. Just feel a bit stupid sometimes as I change my mind often on things I want to sell/keep and I'd just make ya all dizzy I'm sure
Oh yeah if searching for parts it actually works against you if you to leave old ads up. You get a crapload of other sold ads come up. Also I can never figure out why when I do a search, results are all over the map and not chronological.