SELECT 
  cscart_products.*, 
  cscart_products.ebay_status, 
  cscart_products.ebay_template_id 
FROM 
  cscart_products 
WHERE 
  cscart_products.product_id IN (
    21615, 21616, 21617, 21618, 21619, 21620, 
    21621, 21622, 21623, 21624, 21625, 
    21626, 21627, 21628, 21629, 21630
  )

Query time 0.00034

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 (21615,21616,21617,21618,21619,21620,21621,21622,21623,21624,21625,21626,21627,21628,21629,21630)"
    }
  }
}