SELECT 
  cscart_products.*, 
  cscart_products.ebay_status, 
  cscart_products.ebay_template_id 
FROM 
  cscart_products 
WHERE 
  cscart_products.product_id IN (
    20541, 20542, 20543, 20544, 20545, 20546, 
    20547, 20548, 20549, 20550, 20551, 
    20552, 20553, 20554, 20555, 20556
  )

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 (20541,20542,20543,20544,20545,20546,20547,20548,20549,20550,20551,20552,20553,20554,20555,20556)"
    }
  }
}