Japanese e-commerce is fragmented. The same product — a vintage camera, a game console, a brand bag — can vary by ¥5,000–¥30,000 between Amazon.co.jp, Rakuten, Mercari, and Yahoo! Auctions. Finding the delta requires opening four tabs and cross-referencing manually.
I built Arbitra to automate this. Here's the technical side: how to extract pricing data from multiple sites without official APIs, and the patterns that actually hold up across site redesigns.
The Core Problem with DOM Scraping
API-based price tracking is clean: call an endpoint, parse JSON, done. But Japanese e-commerce platforms don't offer public APIs for real-time pricing. Amazon's Product Advertising API requires active affiliate status, Mercari has no public API, and Rakuten's API lags behind displayed prices.
The alternative: content scripts reading the DOM directly.
