I agree that the responsibility is with the seller, but there are occasions when this situation can create confusion. A person may only use the facility once to try and flog something and not return to the site, forget, or just plainly can't be bothered. If the seller has not subscribed to the thread and does not use the site on a frequent basis, the time of interested buyers might be wasted too! Sending a PM may not always bring a reply!
Sorry if I am sounding a little pernickity, but if something has been sold or withdrawn (and not shown on the thread) surely it's not much to ask for the thread to be locked after an appropriate lapse of time?
p.s., only once should a seller be sent a PM as a reminder if the rules of advertising one-off sales are shown (i.e., outlining responsibility and updating information to start).