SELECT 
  cscart_products.*, 
  cscart_products.ebay_status, 
  cscart_products.ebay_template_id 
FROM 
  cscart_products 
WHERE 
  cscart_products.product_id IN (
    21439, 21440, 21441, 21442, 21443, 21444, 
    21445, 21446, 21447, 21448, 21449, 
    21450, 21451, 21452, 21453, 21454
  )

Query time 0.00032

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 (21439,21440,21441,21442,21443,21444,21445,21446,21447,21448,21449,21450,21451,21452,21453,21454)"
    }
  }
}