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It's easier from a moderation point to simply close those threads rather than send PM's which probably won't get answered anyway, I think 60 days is being fairly generous.
If there hasn't been any response in 60 days I think it's fair to say it's gone, there's nothing stopping a member in PM'ing the user in question asking whether it was still for sale.
Why don't you just delete the thread after 60 days to stop all the confusion? Plus it will free up some room on the server. Seems logical to me, but what would I know?