Wow! $2,800 for the 1922 Kenya KUT. Sure hope my family saved all those stamps from my letters in the old days from far flung corners of the world.
That is a 5-pound stamp, equivalent to maybe $250 in today's fluffy money, not something you'd use to mail a letter. Any stamp of that denomination is extremely rare, and it's even more unusual for one to have survived in unused condition.