For the same reason real estate agents are used for buying and selling homes. A domain broker acts as an impartial third party that works to make a deal that both parties are happy with.

A domain broker:

• Knows market value and gives reasonable price expectations to the buyer and seller.
• Handles not so easy to deal with parties or personality conflicts that would otherwise get in the way of a deal.
• Maintains confidentiality of parties so that price in not impacted.
• Takes the guess work out of the whole sales process, setting up escrow, NDAs and assisting with domain transfer.