The GET /buyers API makes it possible to find a buyer by its external_identifier value. 
curl -i -X GET "https://api.example.gr4vy.app/buyers?external_identifier=fe26475d-ec3e-4884-9553-f7356683f7f9" \
    -H "Authorization: Bearer [JWT_TOKEN]"
If no buyers could be found the returned items will be an empty list.
GET /buyers?external_identifier=fe26475d-ec3e-4884-9553-f7356683f7f9
{
  "items": [
    {
      "type": "buyer",
      "id": "fe26475d-ec3e-4884-9553-f7356683f7f9",
      "external_identifier": "412231123",
      "display_name": "John L.",
      "created_at": "2013-07-16T19:23:00.000Z",
      "updated_at": "2013-07-16T19:23:00.000Z"
    }
  ],
  "limit": 100,
  "next_page": null,
  "previous_page": null
}