I don't think they try to sell you what you've alrady bought, I think they're trying to sell you what you have shown any kind of interest in. Perhaps you filled in the order and then backed out- how would Google's recommender system know that? I mean, they sure can but not for everyone, probably (hopefully). So the moment you show any kind of interest in some kind of product, they bombard you with ads for that same kind of product, hoping that they will get you in the short window of time right before you've made a purchase.

... or not. Maybe their recommender systems have simply decided that buying product X is highly correlated with buying product Y where Y = X. Who knows? Probably not even google itself knows.

Also, remember that google is not trying to sell you anything, they're only trying to maximise adoption of their ad platform by advertisers. Who cares what you actually buy? Not Google.