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Good answer Dirk. I like your idea of adding valuable, relevant content to the pages Dirk, good thinking. Personally, I'd rather Iet Google know these pages are removed intentionally and not due to errors, so 410 rather than leaving as 40. One thing to be mindful of, though, is how much crawl budget you're willing to give to these pages. If we're talking about a lot of pages in bulk, I'd be worried how much crawl budget they'd eat up over time. As you point out, they'd likely drop in rank anyway due to loss of internal links too, so might be the cost to the crawl budget isn't worth it?. Another solution (using your idea Dirk), would be to somehow automate the process of, when a listing is marked as sold, the listing is removed, other properties in the same area are added (as you suggest),  then some time later (month or two?), a 410 header set. The other option would be to 301 the old pages back to the area page for the properties (perhaps with something like a bootstrap message saying the property is sold but others in the area are available). This would pass juice etc back to that page. but, of course, you'd be telling G that the page had permanently moved, which isn't quite the case.