I agree with everyone who posted before me, with a twist.
Recently, there was a scam played out here and on pirate4x4 by a user. He would/could manipulate the ads after the money was rec'd and the mods couldnt/wouldnt put it back immediately. So while the complaints were piling up the scammer was basically being 'supported' by the forum system as it is. Caveat: Google caches everything for a long time. IDK how long, but for a long time. So to the people who dont want the ads to be removed, they CAN be found even after deleting or editing to 'kkkkkkk'.
On the other hand, it is a PITA to search for say 'MAC Tools' and find ten posts of only sold items. Why return sold items? Personally, I dont see a use for that in a FS/FT forum. Some have said it is about research, but I think there is/could be a better solution. I think the majority of people searching the classifieds are trying to buy or sell, not research. I have been wrong once, or at least I thought I was. Turned out I was right. (Sarcasm).
I'm not anti-research but I think there could be either a flag for sold, or edit the post as has been suggested. I realize alot of sellers do this, but many dont. The reason for me posting this is to suggest a policy change. I've done it elsewhere and get the 'if it aint broke, dont fix it' line. To me it is broke. It works but not efficiently. If you search my posts, you'll find my diatribe on the subject and a mod that disagrees. My .02
Hope I didnt offend anyone.