I look at this differently.
Personally, I don't think an individual sitting on a Board is not allowed to resign simply because they sold the house before resigning.
The individual was qualified (because he was a member of the Association) and elected to the board.
The individual sold his property and, by doing so, was no longer qualified to serve on the board.
However, the selling of the house did not automatically remove him from the board. It simply made him not qualified to serve.
As a member of the Board, knowing he was no longer qualified to serve, the individual tendered his resignation.
Because he resigned, the Board may appoint someone to fill the vacancy created.
Per the link I posted earlier, it appears that their is split legal opinions on what happens when a member elected to the board is no longer qualified to serve on the board:
"Unqualified Directors
Once a director is no longer qualified to serve on the board, for example, if they cease to be an owner, the board can vacate the person’s seat. (Civil Code § 5105(f)) Once the seat is vacated, the board can appoint someone to fill it (unless the bylaws require that seats be filled by a vote of the membership). See “Director Appointments.” NOTE: There is a split among legal opinions on whether the seat needs to be vacated. Some believe it is legitimate for the director to complete his/her term on the board, but not be eligible thereafter to run for or be appointed to the board."