I am glad you asked - first we confirm the sale actually took place. This helps everyone set a realistic expectation when valuing their portfolio.
Secondly, it helps buyers too: we are able to produce a report for a potential buyer where we illustrate possible price ranges based on certain criteria.
If you have someone interested in a domain and cannot get them to budge on price, having a third party evaluation sent to them can help the buyer better appreciate the need to dig a bit deeper into their pocket. So far we have improved ASP from just over $2200 to just about $3300 for generic domains through this data.
Lastly, the amount of information disclosed is up to the seller. This is why we often report only the length of the domain name or a general category. The exact information is only used to verify and then evaluate the pricing of a given registration.