controller.cs

[SwaggerOperation(
    OperationId = "AvailableForSale",
    Tags = new[] { "Sales" })]
[HttpPost("/sales/products/{sku}/availableForSale")]
public async Task<IActionResult> AvailableForSale([FromRoute] string sku)
{
    var product = await _db.Products.SingleOrDefaultAsync(x => x.Sku == sku);
    if (product == null)
    {
        return NotFound();
    }

    if (product.ForSale)
    {
        return Problem(
            "Product is already Available For Sale.",
            $"/sales/products/{sku}/availableForSale",
            400,
            "Cannot set product as available.",
            "http://example.com/problems/already-available");
    }

    if (product.QuantityOnHand <= 0)
    {
        return Problem(
            "Product has no Quantity on Hand.",
            $"/sales/products/{sku}/availableForSale",
            400, "Cannot set product as available.",
            "http://example.com/problems/no-quantity");
    }

    product.ForSale = true;
    await _db.SaveChangesAsync();

    return Created(_urlHelper.Action("GetSalesProduct", "GetSalesProduct", new { sku }), null);
}