I've always found it very useful when pricing my items on eBay to see the prices for which others have successfully sold theirs with the "Completed Listings" feature. Now, Reverb has added a similar "Sold Listings" checkbox to their search criteria.
As far as I can tell, an item has to have actually been sold on Reverb for it to appear here. If the item was sold elsewhere and its listing removed from Reverb by the seller, you don't see it listed with this feature.