Referly strips out other referrer tags, and we use several approaches to handle "gaming". These are issues that every affiliate network faces, but they can be managed.
You'd be surprised at the level of data the affiliate networks make available to the people using them. Even if you are a small affiliate, purchase/basket level data is on offer.
how could they build a business off of guessing if the purchases are made. especially with the 24 hour length of the cookie, it would make it nearly impossible to track if multiple referly users have the same items. they must have some arrangement or something going on that is not clear.