741, cone sure sure grip, 3.23, 1970. What is it worth?

Rust on gear wear surface makes it a core case and sure grip unit.

Potentially needs additional parts cost of gears, and labor cost of assembly.

$500 minus $200= $300 ...$200 ->actual parts+labor will much be more, but you are getting new gears and fresh setup.

Sure that rust MAY not cause and issue. But the buyer is completely taking a chance with that. Even if it cleans up, there are pits left on the fine surface of the gear.