Why comparison sites are hard
A comparison site is only useful if the same product from twenty merchants is recognised as one product, which merchant feeds actively prevent: inconsistent names, missing identifiers, different specifications and stale availability. Users abandon a comparison site the first time they click through to an out-of-stock item or a price that was not real. Revenue depends entirely on that click actually converting.
What we build for a platform
First, merchant feed ingestion with normalisation, handling the inconsistent formats every merchant sends. Second, product matching across merchants, which is the core value and the hardest engineering problem in the category. Third, availability and price freshness checking, because one dead click loses a user permanently. Fourth, price history and alerts, plus affiliate click tracking with attribution, since accurate attribution is the difference between being paid and arguing with merchants about it.