When the seller registers the domain, they hand over to the domain registrar certain contact information, which is usually entered in the domain's WHOIS record. This information is useful for potential buyers of the domain as it allows them to contact the owner and to verify the owner's identity.
Services exist which allow the domain owner to hide or block their WHOIS information. This can be done for perfectly legitimate reasons but may give the impression that the domain owner has something to hide.
When doing business on Flippa, openness and transparency is important and we recommend that sellers facilitate this by not having privacy enabled on their domains. We furthermore recommend that bidders on these domains do as much due diligence on the seller as possible to verify their authenticity.