I haven't tested their data, but 1 reason why this is hard is due to the SKUs at Walmart and Best Buy. There may be a Samsung 42-inch TV that exists at Amazon, Walmart and Best Buy but have slightly modified specs for each of those mega retailers... and with this, Best Buy has a lot of SKUs that simply die. So once semantics3 has "reconciled" that samsung 42-inch TV across retailers, they'll have to continuously check whether any of the retailers have changed the SKU and/or URL on them.

source: I do this at dealzon.com for a very limited set of data where it's practical