SELECT 
  cscart_products.*, 
  cscart_products.ebay_status, 
  cscart_products.ebay_template_id 
FROM 
  cscart_products 
WHERE 
  cscart_products.product_id IN (
    21551, 21552, 21553, 21554, 21555, 21556, 
    21557, 21558, 21559, 21560, 21561, 
    21562, 21563, 21564, 21565, 21566
  )

Query time 0.00047

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 (21551,21552,21553,21554,21555,21556,21557,21558,21559,21560,21561,21562,21563,21564,21565,21566)"
    }
  }
}