Send mail to product owner with Rules I have a Drupal 7 site running including Commerce. I'm asked to make the webshop work in a way that the mail send after completing payment is send to the product owner(s).

So when creating a product via /admin/commerce/products/add the admin fills in the email address of the product owner/seller in a custom field called field_seller_mail.

When the visitor of the website pays its order, the site should send one or more mails to the owner(s) of the products zo that they can handle the delivery of the products.

In rules I choose the when payed in full trigger.
Then I choose to loop in the action part and loop trough the line items, but the field field_seller_mail is nowhere to be found.

What is the best way to solve this problem?
If the data about a product are not available when processing a "line item", you can make them available. To do so, use a technique similar to what is explained in my answer to "How to access commerce_product field of line-items using Rules to flag the Commerce Product?", which includes a Rules Component that looks like so:

{ "rules_perform_an_action_on_a_selected_line_item" : {
    "LABEL" : "Perform an action on a selected line item",
    "PLUGIN" : "rule",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules" ],
    "USES VARIABLES" : { "selected_line_item" : { "label" : "Selected Line Item", "type" : "commerce_line_item" } },
    "IF" : [
      { "entity_has_field" : { "entity" : [ "selected-line-item" ], "field" : "commerce_product" } }
    ],
    "DO" : [
      { "entity_fetch" : {
          "USING" : {
            "type" : "commerce_product",
            "id" : [ "selected-line-item:commerce-product:product-id" ]
          },
          "PROVIDE" : { "entity_fetched" : { "product_fetched" : "Fetched Product" } }
        }
      },
      { "drupal_message" : { "message" : "Some details about this product: Product ID = [product-fetched:product-id], Product SKU = [product-fetched:sku], Product Title = [product-fetched:title], Product Creator = [product-fetched:creator]" } }
    ]
  }
}


Here is a blueprint of what you need to do:


Create a Rules Component, similar to the above Rules Component. Just an import of that Rules Component should be enough (to get started and further improve it as detailed below).
Rework the Drupal Message shown via the last Rules Action in the Rules Component (Starting with "Some details about this product"), e.g. by also using any of the available tokens that fit your needs.
Execute that Rules Component inside your Rules Loop of the rule in which you want to loop over all line items.
Improve the Rules Component from Step 1 by creating whatever additional Rules Action(s) you need. Make sure to place these Rules Action(s) AFTER that Rules Action (in the Rules Component) that performs "Fetch entity by ID".