Trying to compute "where is this product cheapest" across Rakuten's and Yahoo!'s APIs, you hit a wall before any price math: how do you decide two listings are the same product? The JAN (the product's barcode number) would make it easy — except the two marketplaces are asymmetric about it.
Yahoo! Shopping: a jan_code parameter looks the JAN up directly, and the response carries janCode back.
Rakuten item search: no JAN parameter. You throw the JAN string into keyword, and the JAN itself usually sits inside the item caption rather than a structured field.
So Yahoo! you can trust into a confirmed JAN match; Rakuten you can't use without proving that a keyword=JAN hit is really that product. Here's how I split matching into three certainty tiers around that.
Split identity into three certainty tiers
