SELECT 
  cscart_products.*, 
  cscart_products.ebay_status, 
  cscart_products.ebay_template_id 
FROM 
  cscart_products 
WHERE 
  cscart_products.product_id IN (
    21327, 21328, 21329, 21330, 21331, 21332, 
    21333, 21334, 21335, 21336, 21337, 
    21338, 21339, 21340, 21341, 21342
  )

Query time 0.00054

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 (21327,21328,21329,21330,21331,21332,21333,21334,21335,21336,21337,21338,21339,21340,21341,21342)"
    }
  }
}