How to access all of the properties related to Commerce Line Item entity in a rule?
Below is a possible approach you can use to solve your question. It provides the details about how I was able to have a set of Drupal messages being shown at checkout time, which look similar to this testcase I used (for a cart with 3 line items):

Current line item has label 'USB-BLU-08' and id '17'
Current line item has label 'TSH4-BLK-MD' and id '18'
Current line item has label 'MES1-BLU-OS' and id '19'


To get this to work, I created a custom rule, which looks like so (use the Rules UI to import it in your own site):

{ "rules_access_line_items_at_checkout" : {
    "LABEL" : "Access line items at checkout",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules", "commerce_checkout" ],
    "ON" : { "commerce_checkout_complete" : [] },
    "DO" : [
      { "LOOP" : {
          "USING" : { "list" : [ "commerce-order:commerce-line-items" ] },
          "ITEM" : { "line_item" : "Current line item" },
          "DO" : [
            { "drupal_message" : { "message" : "Current line item has label \u0027[line-item:line-item-label]\u0027 and id \u0027[line-item:line-item-id]\u0027" } }
          ]
        }
      }
    ]
  }
}


Some details about it:


Rules Event: After completing the checkout process.
Rules Condition: none.
Rules Action: Create a loop, to iterate over each line item. And for each line item, show a Drupal message, starting with "Current line item has ...". Obviously, at this point you could display any of the properties related to a line item.


Easy, no?

PS: I'm assuming this question is about D7 ...