Etsy API updateInventory responds with 200 OK, but doesn't update inventory data

I have an Etsy listing with 1 product in draft state. When I try to update the inventory of this listing through the updateInventory API endpoint (docs: https://www.etsy.com/developers/documentation/reference/listing#method_updatelisting) I get errors.

I followed the example in 'Updating a Listing with one product' section to try to update the price, quantity, and SKU with no luck: https://www.etsy.com/developers/documentation/getting_started/inventory

I get either an oauth signature_invalid error, or an expected products property to be present error.

Finally, I was able to make a request that returned a 200 OK. But no data in the listing was updated. Here is that request (I added new-lines in the body for readability, the original request has no spaces or new-lines in the body):

POST https://openapi.etsy.com/v2/listings/808984070/inventory?method=PUT HTTP/1.1
Host: openapi.etsy.com
Authorization: OAuth oauth_consumer_key="xxxxxxxxx",oauth_nonce="xxxxxxxxx",oauth_signature="xxxxxxxxx",oauth_signature_method="HMAC-SHA1",oauth_timestamp="1591886701",oauth_token="xxxxxxxxx",oauth_version="1.0"
Accept: application/json, text/json, text/x-json, text/javascript, application/xml, text/xml
User-Agent: RestSharp/106.11.4.0
Connection: Keep-Alive
Request-Id: |38197ee2-416cf452906ba6a9.4.
Accept-Encoding: gzip, deflate
Content-Type: application/json
Content-Length: 192

{
  "products": [{
    "product_id": 4415387567,
    "sku": "TEST1",
    "property_values": [],
    "offerings":[{
      "offering_id": 4627746384,
      "price": "50.00",
      "quantity":100,
      "is_enabled":1,
      "is_deleted":0
    }],
    "is_deleted":0
  }]
}

Why is Etsy not updating the inventory, even though it returns a 200 OK? More specifically, what should change in this request to get Etsy to update the inventory data?

P.S. Some of the things I tried before arriving at the above request:

Tried sending a PUT request without a "?method=PUT" in the URL. Got a signature_invalid error.
Tried using x-www-form-urlencoded formatting for the body by setting the Content-Type header to that value, and setting the body to products=<url-encoded string of JSON representation of products array>. Got a signature_invalid error.
Tried with and without a SKU - I get back a 200 OK, but SKU doesn't get set.

Solution

The issue stemmed from my misunderstanding how to use the C# library that I was using (RestSharp). After some tweaks, I was able to generate a request that works. Here is what it looks like:

PUT https://openapi.etsy.com/v2/listings/834145978/inventory HTTP/1.1
Host: openapi.etsy.com
Authorization: OAuth oauth_consumer_key="xxxxx",oauth_nonce="xxxxx",oauth_signature="xxxxx",oauth_signature_method="HMAC-SHA1",oauth_timestamp="xxxxx",oauth_token="xxxxx",oauth_version="1.0"
Accept: application/json, text/json, text/x-json, text/javascript, application/xml, text/xml
User-Agent: RestSharp/106.11.4.0
Connection: Keep-Alive
Request-Id: |feb37503-4e0cc9e4a34b242a.4.
Accept-Encoding: gzip, deflate
Content-Type: application/x-www-form-urlencoded
Content-Length: 654

products=%5B%0D%0A%20%20%7B%0D%0A%20%20%20%20%22product_id%22%3A%204638831553%2C%0D%0A%20%20%20%20%22sku%22%3A%20%22N27%22%2C%0D%0A%20%20%20%20%22property_values%22%3A%20%5B%5D%2C%0D%0A%20%20%20%20%22offerings%22%3A%20%5B%0D%0A%20%20%20%20%20%20%7B%0D%0A%20%20%20%20%20%20%20%20%22offering_id%22%3A%204740581353%2C%0D%0A%20%20%20%20%20%20%20%20%22price%22%3A%20%2265.00%22%2C%0D%0A%20%20%20%20%20%20%20%20%22quantity%22%3A%201%2C%0D%0A%20%20%20%20%20%20%20%20%22is_enabled%22%3A%201%2C%0D%0A%20%20%20%20%20%20%20%20%22is_deleted%22%3A%200%0D%0A%20%20%20%20%20%20%7D%0D%0A%20%20%20%20%5D%2C%0D%0A%20%20%20%20%22is_deleted%22%3A%200%0D%0A%20%20%7D%0D%0A%5D

The parts to note are:

the request method is PUT
the Content-Type header is application/x-www-form-urlencoded
here is what the request body looks like before being url-encoded for the request:

products=[
  {
    "product_id": 4638831553,
    "sku": "N27",
    "property_values": [],
    "offerings": [
      {
        "offering_id": 4740581353,
        "price": "65.00",
        "quantity": 1,
        "is_enabled": 1,
        "is_deleted": 0
      }
    ],
    "is_deleted": 0
  }
]

Not exactly related, but here is the code I use to create a new listing in Etsy, using C# and RestSharp. This is a work-in-progress and needs cleanup, but it works:

public async Task<ListingModel> PostListing(ListingCreateModel createModel)
{
    // Create authenticator for the URL we will need to use.
    _restClient.Authenticator = MakeAuthenticatorForProtectedResource();

    // Update the create model, making sure we set the ShippingProfileId - this way
    // you won't need to set it manually in every listing afterwards.
    createModel.Quantity = createModel.Quantity;
    createModel.ShippingProfileId = ShippingProfileId;

    // Create a basic listing first, with minimal properties.
    var basicListing = ExecuteAndDeserialize<ListingListModel>(
        CreatePostRequest("listings", createModel)).Results.First();

    // Get the current products for this listing (we'll need to modify that object
    // and send it back to Etsy.)
    var productList = ExecuteAndDeserialize<dynamic>(
        CreateGetRequest($"listings/{basicListing.ListingId}/inventory"));
    
    // set SKU, price, and quantity (just experimenting, you can set any
    // supported properties you need here)
    productList.results.products[0].sku = createModel.Sku;
    productList.results.products[0].offerings[0].price =
        createModel.Price.ToString("0.00");
    productList.results.products[0].offerings[0].quantity =
        createModel.Quantity;

    // Now that we have an updated products list, call the PUT endpoint to
    // update the listing.
    // The CreatePutRequest method sets the 'Content-Type' header to
    // 'application/x-www-form-urlencoded'.
    // RestSharp will take care of serializing the `new { productList.results.products }`
    // object to the form-urlencoded format.
    var updatedListing = ExecuteAndDeserialize<dynamic>(
        await CreatePutRequest($"listings/{basicListing.ListingId}/inventory",
        new
        {
            productList.results.products
        }));
    
    var newListingList = ExecuteAndDeserialize<ListingListModel>(
        CreateGetRequest($"listings/{basicListing.ListingId}"));
    return newListingList.Results.First();
}

private async Task<IRestRequest> CreatePutRequest<TBody>(string resource, TBody body)
{
    var request = new RestRequest(resource)
    {
        Method = Method.PUT
    };

    request.AddHeader("Content-Type", "application/x-www-form-urlencoded");
    request.AddObject(body);
    return request;
}

//-------- NOT SO RELEVANT TO THE QUESTION ----------

private IRestRequest CreateGetRequest(string resource)
{
    var request = new RestRequest(resource)
    {
        Method = Method.GET
    };

    return request;
}

private OAuth1Authenticator MakeAuthenticatorForProtectedResource()
{
    var auth = OAuth1Authenticator
        .ForProtectedResource(_authRepo.AuthInfo.AppKey, _authRepo.AuthInfo.SharedSecret,
        _authRepo.AuthInfo.AccessToken, _authRepo.AuthInfo.AccessTokenSecret);
    return auth;
}

private T ExecuteAndDeserialize<T>(IRestRequest req)
{
    var response = _restClient.Execute(req);
    if (response.IsSuccessful)
    {
        return JsonConvert.DeserializeObject<T>(response.Content);
    }

    if ((int) response.StatusCode >= 400 && (int) response.StatusCode < 500)
    {
        throw new ValidationException(response.Content);
    }
    throw new InvalidOperationException(response.Content);
}