Marketplacer variant stock

First, we create a cart:
```
mutation cartCreate($input: CartCreateMutationInput!) {
  cartCreate(input: $input) {
    cart {
      ...cartInfo
    }
    clientMutationId
    status
    errors {
      ...errorInfo
    }
  }
}
```


Result: 
```
{
  "data": {
    "cartCreate": {
      "cart": {
        "id": "Q2FydC0zNTEwOTM1",
        "token": "3510935-b4501f072f0cddf9e2de7612cc80dbe4",
        "buyableQuantity": 0,
        "cartSellers": {
          "edges": []
        }
      },
      "clientMutationId": "12345",
      "status": 200,
      "errors": null
    }
  }
}
```

Then we retrieve the variant `VmFyaWFudC02MTc4OQ==`:

```

fragment variantQL on Variant {
  id
  displayable
  label
  countOnHand
  externalId
  infiniteQuantity
  lowestPriceCents
  lowestOriginalPriceCents
  barcode
}

# Queries all variants for the given IDs.
query getVariants($ids: [ID!]!) {
  nodes(ids: $ids) {
    ...variantQL
  }
}
```

The result says that the item is displayable and that there are 118 on hand. 
```
{
  "data": {
    "nodes": [
      {
        "id": "VmFyaWFudC02MTc4OQ==",
        "displayable": true,
        "label": "6' 6\" / Medium Heavy / Fast / N/A / 12-20lb / 1 pc / N/A",
        "countOnHand": 118,
        "externalId": "1398296",
        "infiniteQuantity": false,
        "lowestPriceCents": 7995,
        "lowestOriginalPriceCents": 7995,
        "barcode": "036282324923"
      }
    ]
  }
}
```

However, trying to add the item to the cart results in an error:
```
{
  "data": {
    "cartAddCartItem": {
      "cart": null,
      "clientMutationId": "123",
      "status": 200,
      "errors": [
        {
          "field": "out_of_stock",
          "messages": [
            "variant is out of stock"
          ]
        }
      ]
    }
  }
}
```