Mr. Neil Agate, the seller of the Sussex.com domain name to the Duke and Duchess of Sussex, was unaware of the royal buyers' identity.
This fortunate entrepreneur and British expatriate has held onto the domain name since registering it in 1995, using it for his company, Sussex Systems Inc.

A business card for Sussex.com – Photo sourced from Kathy McCarl Agate on Facebook
In December of last year, a third-party broker contacted the former owner of Sussex.com, who then conveyed his asking price to the unidentified buyer.
Speaking to the Daily Mail, Mr. Neil Agate stated that the price was "fair," and the transaction was completed in December 2023.
From public posts on his Facebook profile, it's evident that Mr. Agate was a very pleased seller:

Post content: Congratulations to the Duke and Duchess of Sussex on the purchase of Sussex.com for your new website. This domain has served me well for the past 30 years, and I'm particularly pleased it's still owned by a British player. (By the way, my sussexcom email address is no longer valid.)