How to make Drupal Commerce send email notification to the author of products sold?
In short; you should loop over all commerce order line items, obtain the product owner and add it to a list. Then in a new loop, go through the created list to send all product owners a main.

First you need a rules component that checks if a provided line item contains a product field and pushes the author to a given list (enforcing uniqueness makes sure every product owner is only added once). You can import below code on the components tab:

{ "rules_product_on_line_item" : {
    "LABEL" : "get product owner from line item",
    "PLUGIN" : "rule",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules" ],
    "USES VARIABLES" : {
      "line_item" : { "label" : "Line item", "type" : "commerce_line_item" },
      "owners" : { "label" : "Product owners", "type" : "list\u003Cuser\u003E" }
    },
    "IF" : [
      { "entity_has_field" : { "entity" : [ "line-item" ], "field" : "commerce_product" } }
    ],
    "DO" : [
      { "list_add" : {
          "list" : [ "owners" ],
          "item" : [ "line-item:commerce-product:creator" ],
          "unique" : "1"
        }
      }
    ]
  }
}


Now for the main rule, create a new variable of type list of users. This is where we will store all product owners in.

Create a loop over commerce-order:commerce-line-items. In this loop, use the component that was created above. This will fill the list of product owners.

Finally, loop over the list of product owners, and send each of them a mail. The complete rule would look something like this:

{ "rules_mail_all_product_owners" : {
    "LABEL" : "Mail all product owners on an order",
    "PLUGIN" : "reaction rule",
    "WEIGHT" : "5",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules", "mimemail", "commerce_checkout" ],
    "ON" : { "commerce_checkout_complete" : [] },
    "DO" : [
      { "variable_add" : {
          "USING" : { "type" : "list\u003Cuser\u003E" },
          "PROVIDE" : { "variable_added" : { "product_owner_list" : "Product owner list" } }
        }
      },
      { "LOOP" : {
          "USING" : { "list" : [ "commerce-order:commerce-line-items" ] },
          "ITEM" : { "list_item" : "Current list item" },
          "DO" : [
            { "component_rules_product_on_line_item" : { "line_item" : [ "list-item" ], "owners" : [ "product-owner-list" ] } }
          ]
        }
      },
      { "LOOP" : {
          "USING" : { "list" : [ "product-owner-list" ] },
          "ITEM" : { "productowner" : "Current product owner" },
          "DO" : [
            { "mimemail" : {
                "key" : "mailkey",
                "to" : [ "productowner:mail" ],
                "from_mail" : [ "site:mail" ],
                "subject" : "",
                "body" : ""
              }
            }
          ]
        }
      }
    ]
  }
}