Worthpoint is not a sales site.
They offer a subscription service to their aggregation of sales of many types of collectibles. As such, they provide a service similar to what Stamp Auction Network is now offering with their subscription to "prices realized".
They gather their data from eBay
, many philatelic auctions, and other "estate" style and collectibles auctions.
As to the value of their data, I have no information as I have never had a subscription.