SELECT 
  cscart_products.*, 
  cscart_products.ebay_status, 
  cscart_products.ebay_template_id 
FROM 
  cscart_products 
WHERE 
  cscart_products.product_id IN (
    21343, 21344, 21345, 21346, 21347, 21348, 
    21349, 21350, 21351, 21352, 21353, 
    21354, 21355, 21356, 21357, 21358
  )

Query time 0.00039

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 (21343,21344,21345,21346,21347,21348,21349,21350,21351,21352,21353,21354,21355,21356,21357,21358)"
    }
  }
}