I noticed diesel isnt listed in outpost 
so I tried to add it :

{
        "_comment": "Sell Low Grade x 300 for Diesel x 1",
        "sellId": -946369541,
        "sellAmount": 300,
        "sellAsBP": false,
        "currencyId": -1568388703,
        "currencyAmount": 500,
        "currencyAsBP": false
      },​

But its still not showinng (after plugin reload).

Any advice? 

You get it figured out? If not, you need to remove the - in front of the diesel ID. Its 1568388703.