SELECT 
  cscart_products.*, 
  cscart_products.ebay_status, 
  cscart_products.ebay_template_id 
FROM 
  cscart_products 
WHERE 
  cscart_products.product_id IN (
    21407, 21408, 21409, 21410, 21411, 21412, 
    21413, 21414, 21415, 21416, 21417, 
    21418, 21419, 21420, 21421, 21422
  )

Query time 0.00033

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 (21407,21408,21409,21410,21411,21412,21413,21414,21415,21416,21417,21418,21419,21420,21421,21422)"
    }
  }
}