Well, Apple does not actually know which serial numbers have been sold. You can still buy Apple products outside of an Apple store, and even at an Apple Store, knowing which serials are sold is logistically difficult.
They could probably implement a process at Apple stores but to do it for third party stores would be more difficult. To avoid simply adding a step for a user to claim a serial number as sold when they set up their Hackintosh or HackiPhone (can we coin this?), you'd probably need a way to authenticate as the store when you mark one off. But there's nothing stopping stores who have excess inventory from selling it off at cost or even a discount when a particular product isn't doing well- and at that point those purchasers will need to be able to mark off serial numbers when they sell those devices.
It's not impossible but it's not terribly likely, at least any time soon.