In an ideal world, but not in this one where profit is king. Some chains are worse than others for this. DSG Retail(*) are, IMHO, the worst offenders. I have bought two items shrink wrapped, sealed, "as new" from DSG Retail stores that have been faulty and obvious customer returns on opening.
Why should the manufacturer foot the bill. The goods as supplied by them where not faulty and complete. The store has to take the loss, how they deal with that is up to the store. As you say the incomplete product should not go back on the shelf "as new", about the only option they have is to put it in the "reduced" but it really needs to be labeled "incomplete" as well.
Oh, you do know what being assertive is rather than agressive. B-)
(*) DSG Retail = Dixons, The Link, PC World, Elkjop, UniEuro, Currys, Dixons Group Business Services, Partmaster Direct, Mastercare, pc service call, PC World Business, PC City & Electro world.
Bung "DSG Retail" into google and see the great long list of court cases, complaints consumer support groups...