Once once they are rejected by charities, they can be destroyed just like before this law. EU regulators are flawed humans, but you can be assured that any argument you can come up with in 10 minutes has been considered.
Those are used clothes. New clothes, especially from a brand that would use a shredder to have a little more exclusivity? I expect almost all to sell just fine.
Market clearing prices can be negative i.e. you have to pay someone to "buy" them. This regularly happens in commodities markets. In agriculture, for example, negative market clearing prices is why a lot of food ends up in a landfill instead of on store shelves.