"Handle with gloves" Except for modern mirror finish proof coins, I don't think it matters all that much if you use common sense, and handle all coins on the edge, as shown in the OP's link. I'll bet that handling that way made no difference to the sale price.
As for the airplane stamp, I know of it, and it isn't the first to show a vignette upside down, eg the upside down Black Swan of Western Australia (also rare). I know the the dark mauve British Guiana stamp is famous, but I think it is a bit of a fizzer.
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