This cannot be stressed enough. I can't tell you how many useless things websites have suggested I would be interested in because I purchased a gift for someone

This cannot be stressed enough. I can't tell you how many useless things websites have suggested I would be interested in because I purchased a gift for someone's kid 5 years ago, or whatever. Also, consider that data isn't necessarily properly associated with who you think it is. I often see suggestions that are clearly due to my wife's browsing on her own computer (which is nearly identical to mine in terms of browser fingerprinting). Obviously we have the same IP to the outside world and we share some interests, but not everything!