How about a new section for all closed sale threads - An archive section.
The other forum I go on works like this:
1. You can only create 1 for sale/wanted thread in each section at any one time.
2. Once the item has sold, or you remove it from sale etc, you have to archive the thread (there is an option to archive alongside close thread etc).
3. This locks and archives the thread together, moving it to the archived section and free'ing you up to create a new sale thread in that section again.
The above ensures that all for sals threads are current and available, since if they were not they would be in a separate archive section.
If we decided to lock all for sale threads here so no-one could post, any interested parties would have to ask all questions (questions that others may also be thinking of) via PM. This means you may be answering the same continually. Although sometimes people can mess up your sale thread, most of the time members may ask questions that clarify aspects of the item etc, and are therefore useful for others to see, and may actually help sell the item.
Hence, personally I think they should be left open, and you simply report any posts that are irrelevant so a mod can remove them for you.