It seems like a fundamentally difficult problem to know when you've already bought something. Obviously, it's solvable in simple cases, like when you browse an

It seems like a fundamentally difficult problem to know when you've already bought something. Obviously, it's solvable in simple cases, like when you browse and buy from the same vendor. But in other cases, it's not, like if you browse around multiple places and buy from one, or if you look around online and buy in person. Basically, the information that you're interested in buying is going to spread around to certain places, and the information that you've bought is also going to spread around, but there is nothing to guarantee that it will percolate to all the same places. You also have to consider the trade-off an advertiser faces. They'd probably rather show an irrelevant (already bought) ad than miss a chance to show a relevant one to someone who is still actively shopping. So they'd probably choose to err on that side.