If you're too sorry to do such a simple task as deleting your ads after your items sale then I can only imagine how sorry you are at everything else you do in life. Anyone guilty of this should be banned from the site.
Sometimes you need to take into consideration that items "sold" aren't always really "sold" even though the seller might tell you they already have a buyer for the item listed that you are inquiring about. ...Confused yet? Let me explain...
The problem as a seller who posted an ad is someone comes along and tells you they want to buy your item from you and yes, I suppose following your logic, this SHOULD be the time when you should mark the ad item as "sold" right??.... um.. no..not exactly, ...I am afraid not, ...and let me explain why.
What often happens in thses situations is the person who expresses an interest to buy you item, never actually sends you any payment for the item listed in the ad. (happened to me on many occasions) However, since you already marked the item as sold on the listed ad, you don't get to make the sale NOR do you get any more opportunity or interest in the item from anyone else for that matter because the listing is marked as "sold" even though you never received payment from anyone.
Therefore "sold" in my opinion is when I receive actual payment for the item in my hand, not when someone who expressed the desire and "intention" to purchase the item from me. Therefore, until payment is received, then items listed will remain "for sale." and not marked as "sold" I don't consider this approach to be "irresponsible" per se, but rather it leaves the item up for sale, and the ongoing opportunity to keep the item posted for sale until actual payment is actually received for the item. I highly suspect this is probobly the reason behind why many others never mark the ads as sold either.
I agree it SHOULD be a simple task, but the issue remains that way because no one is really sure what defines "sold", when selling actually occurs and because of this, it sometimes becomes a complex issue involving some of the listed ads.
Now rather than mark the item as sold, I will usually try to mark the ad as "pending payment" until payment is received. (not always an option available to do this!!. Sometimes your only option is to either mark it as sold or delete the ad in its entirety.)
I think it would be better to have a 3 step process in the ad listing process. One is obviously called "For Sale" 2. Pending Payment and 3. Sold !!
I feel this approach would resolve a lot of the problems and confusion potential buyers and sellers alike often experience.
I don't agree with time sensitive expired ads approach because some items can take many months to move especially if the item isn't popular or its not a particularly highly desired item or part. (non standard radio crystals or similar low demand items come to mind.)