> Even if the retailer took it back it would still be... trash.

Yes, but making them deal with it would create a massive incentive to either reduce the amount of rubbish they make, or to make it recyclable/processable.

It's an externality because the entity that sold it to you doesn't have to pay the consequences of dealing with the trash. OP said "dispose of it properly," which could mean a lot of things, all of which are better than leaving it on a beach.

It should be at a minimum stored safely. How and why are the environmental effects not a factor for you?