how can I know an Apple Pay receipt is valid?

Recently I encountered a problem. Someone is selling our app's service (in-app purchase) in the Internet, which is a bit cheaper than our price.

I have checked the receipt received from client through the api of Apple server (https://buy.itunes.apple.com/verifyReceipt):

{
  "receipt": {
  "app_item_id": "123456",
  "bid": "com.aaa.bbb",
  "bvrs": "2.11.11",
  "is_in_intro_offer_period": "false",
  "is_trial_period": "false",
  "item_id": "123456",
  "original_purchase_date": "2018-05-14 02:13:01 Etc/GMT",
  "original_purchase_date_ms": "1526263981276",
  "original_purchase_date_pst": "2018-05-13 19:13:01 America/Los_Angeles",
  "original_transaction_id": "90000426694387",
  "product_id": "char1200",
  "purchase_date": "2018-05-14 02:13:01 Etc/GMT",
  "purchase_date_ms": "1526263981276",
  "purchase_date_pst": "2018-05-13 19:13:01 America/Los_Angeles",
  "quantity": "1",
  "transaction_id": "123456726694387",
  "unique_identifier": "abcdeffb70076be1430c754569fd1fba2f1546bc",
  "unique_vendor_identifier": "ABCDEF-ABCD-4D86-824B-840CFF57D6FF",
  "version_external_identifier": "123476292"
  },
  "status": 0
}

I can't find anything wrong. I have listed the questions below:

  1. Could someone tell me how can I check the validation of the purchase directly?
  2. If the money is not payed into my account, how can I check this situation?
  3. Why can others buy our service in a lower price?