Evidently, they have a big adjacency matrix somewhere, one column for every customer and one row for every purchase, and quite possibly they're running some sort of a graph diffusion on it.

My information might be out of date, but last time I checked they had one such matrix per department, supposedly with some timing information etc. So they won't recommend Bolt plush toy if you liked Bolt DVD.

I'll leave the moral of this story to you.