"If I buy a mattress, advertising more mattresses to me doesn't make me more likely to buy them; I bought the thing already."
Actually, it is possible that you might want to buy a mattress again. Consider that you found the new mattress to be more/less firm and are looking to return it and get a different one. I think it is reasonable to show ads for the same thing at least till the end of return window.
You may get extreme edge cases, but you're going to pay more to get them because the conversion rate will be low. You'd get more out of targeting placements directly because you would hit all people looking for a new mattress there, not just people that have already purchased one.
Actually, it is possible that you might want to buy a mattress again. Consider that you found the new mattress to be more/less firm and are looking to return it and get a different one. I think it is reasonable to show ads for the same thing at least till the end of return window.