At Ranch/stable you can only buy the old Horse saddle. After the update they added 2 new items: Single and double horse saddle. I guess the plugin still using the orgianel Horse saddle that dont longer work. the old horse saddle is not possible to use.

I fixed it with changing data file to this:

{
"_comment": "Sell Single Horse Saddle x 1 for Scrap x 75",
"sellId": 1559915778,
"sellAmount": 75,
"sellAsBP": false,
"currencyId": -932201673,
"currencyAmount": 1,
"currencyAsBP": false,
"weight": 100,
"refillAmount": 100000,
"refillDelay": 0.0
},
{
"_comment": "Sell Double Horse Saddle x 1 for Scrap x 75",
"sellId": -1323101799,
"sellAmount": 90,
"sellAsBP": false,
"currencyId": -932201673,
"currencyAmount": 1,
"currencyAsBP": false,
"weight": 100,
"refillAmount": 100000,
"refillDelay": 0.0
},

I keep getting (IndexOutOfRangeException: Index was outside the bounds of the array.) when I put that into the data file. Do I just need to restart the server with the new data file in?

Seezure

I keep getting (IndexOutOfRangeException: Index was outside the bounds of the array.) when I put that into the data file. Do I just need to restart the server with the new data file in?

Yes you need to unload, save the data and restart