code: protected function getEbayHeaders()
      {
          $headers = [];
  
          // Add required headers first.
          $headers[self::HDR_AUTHORIZATION] = 'Bearer '.$this->getConfig('authorization');
  
          // Add optional headers.
          if ($this->getConfig('marketplaceId')) {
              $headers[self::HDR_MARKETPLACE_ID] = $this->getConfig('marketplaceId');
          }
  
          return $headers;
      }
docs: Builds the needed eBay HTTP headers.
  
  @return array An associative array of eBay HTTP headers.