If you want to search for only closed trades type: "nsfw:yes aaaa" where aaaa is the skin that you're looking for. If you wanted to find Sapphires in closed posts from the past week, I recommend searching "nsfw:yes sapphire" and setting the search to relevance and past week (past month/past year if not enough results. If you wanted only karambits I would do "nsfw:yes karambit sapphire" with the same settings. You're welcome and good luck updating prices.
-OceanManSam
PS: Many sold items have prices in posts that are not closed (such as posts with multiple items and stores) so I don't always recommend this method.