SELECT 
  cscart_products.*, 
  cscart_products.ebay_status, 
  cscart_products.ebay_template_id 
FROM 
  cscart_products 
WHERE 
  cscart_products.product_id IN (
    21951, 21952, 21953, 21954, 21955, 21956, 
    21957, 21958, 21959, 21960, 21961, 
    21962, 21963, 21964, 21965, 21966
  )

Query time 0.00043

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 (21951,21952,21953,21954,21955,21956,21957,21958,21959,21960,21961,21962,21963,21964,21965,21966)"
    }
  }
}