To be perfectly honest, the history of that coin in my opinion is completely irrelevant. Not that i'm doubting the person listing it, but unless you can speak to the African that found it (or better still, get him to sign a piece of paper describing where and when he found it etc) then there is no way you can guarantee it was found where it says it was. The African gentleman who bought it at a boot sale in Harare (or wherever) could have sold it to the Englishman telling him he found it by a specific river in order to sell it for more. The African gentleman could have even claimed it came out of the belly of a wild Tiger that he shot while it was attacking Michael Caine during the filming of Zulu if he wanted to! 1836 was a long time ago, who knows where the coin has been since then. Maybe it was dropped by the river a week earlier by someone on their way back from the previous weeks boot sale! What I'm saying is, buy it if you like the look of it, but take the history with a pinch of salt. Removal of the stain will depend on what it is, I also think the stain should stay as it would be better than a hairlined cleaned surface, and any future keeper of the coin would have the option of cleaning it should they want to.