I'm sure this topic should probably be split off into its own thread (out of a trading post thread for a sale that has already been completed successfully), but I thought this worth a reply as I think the topic of conversation is a good one.
For the record, I'm in agreement on the opinion as to keeping the sale ad in tact - pricing and all - after the sale; I've said as much here on AC in the past, and I will again point out that from a perspective of adding value to AC as a website, this information is valuable and arguably some of the only long-term value that may come to AC as a website from hosting a free to the sellers selling market (that most everywhere else comparable charges a premium for, I might add) that does nothing but cost AC in administrative resources and bandwidth.
That aside, from experience seeing this sort of setup on websites in industries/hobbies outside of this one, there are two reasons that immediately come to mind that I've seen/read/heard of sellers deleting selling prices (or whole ads):
One, some people feel that by doing so they are allowing the sellers that follow them a free reign to set their prices at whatever they choose and get as much for what they are selling as the market at the time will pay, without influence from an ad-hoc "market valuation" of second hand products; more or less making an auction-esque environment from an non-auction selling market.
Two, from a self-centered perspective, if a seller deletes the selling price and or the whole add after concluding the transaction, there is less or no chance that people will necro-post or send random PMs from now until the end of time asking if an item is still for sale when a Google search brings up an ad that may be years old when someone looking to buy a thing goes out looking for one. There is only one web forum I've visited that I can think of that has come up with an orderly and elegant solution to this problem, but doing so required customizing a chunk of the forum website code such that it might not be practical for someone running an online forum that didn't also want to manage/babysit custom code from then on until the website goes dark forever.
In short, while I agree with the sentiment that keeping the prices and ads intact for future reference is at the very least doing a little to give back to an online community that allowed one to sell their stuff for free, there are some valid reasons for folks doing what they do otherwise.
