That is who is organizing the auction though, not who the proceeds are going to. The production company is what owns the property in terms of legal title, they are the ones that actually went out to commission/buy the specific items. But the beneficial interest is with Netflix/the distributor, who funded the project to the production company (along with any other funders, they all get a pro rata share of the returns).

Of course, Marvel could have contracted to keep it. We don't know the details and they could come to any arrangement they wanted for the most part. But the production company selling it is pretty standard situation.

For example in the car insurance analogy above, under some insurance contracts you have the opportunity to sell the salvage yourself, as long as the money goes to the insurance company. (And if you sell the salvage unilaterally, the insurer can sue you to claim what you sold it for). Insurers don't like that though because the amount you agree to might not be what they think they can get for it.