SELECT 
  cscart_products.*, 
  cscart_products.ebay_status, 
  cscart_products.ebay_template_id 
FROM 
  cscart_products 
WHERE 
  cscart_products.product_id IN (
    21535, 21536, 21537, 21538, 21539, 21540, 
    21541, 21542, 21543, 21544, 21545, 
    21546, 21547, 21548, 21549, 21550
  )

Query time 0.00042

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_products",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["product_id"],
      "rows": 16,
      "filtered": 100,
      "index_condition": "cscart_products.product_id in (21535,21536,21537,21538,21539,21540,21541,21542,21543,21544,21545,21546,21547,21548,21549,21550)"
    }
  }
}