Unable to purchase assets through the API

Im attempting to purchase a Roblox item through the economy API, this one:
https://economy.roblox.com/v1/purchases/products/:productid:

Item im testing it on:

My JSON body im sending:

{
    "expectedCurrency": 0,
    "expectedPrice": 5000,
    "expectedSellerId": 1
}

In the headers I’ve included my cookie and x-csrf-token

When I do make a post request to the endpoint with the right productid & body it ends up returning the following:

{
    "errors": [
        {
            "code": 0,
            "message": "InternalServerError"
        }
    ]
}

From what I know this is the API roblox has been using for purchases.

Expected Output:

{
    "purchased": true,
    "reason": "Success",
    "productId": 1876986114,
    "currency": 1,
    "price": 120,
    "assetId": 186956940,
    "assetName": "Imperfectly Illegitimate Businessman",
    "assetType": "Hat",
    "assetTypeDisplayName": "Hat",
    "assetIsWearable": true,
    "sellerName": "Roblox",
    "transactionVerb": "bought",
    "isMultiPrivateSale": false
}