For me this ethical conundrum boils down to simple rule of thumb: Substitute animal with a human.
Would you wear a second hand human leather jacket or drive a human leather seats car?
This is why I will probably never wear my beloved leather jacket again.
I agree that the substitution method is a great and simple way for answering ethical questions.
However, I might ask you this:
If our 'moral community' (or in other words the group of beings which we consider to be ethically relevant and which 'matter') is determined by asking 'is such a being sentient?' then how can we include animals that have long been dead into that community when dead animals are most certainly not sentient?
My point being, yes your question "Would you wear a second hand human leather jacket or drive a human leather seats car?" is worth asking, but is it truly an ethical question if it concerns dead beings?
Sure, I also do feel that way, but I think it is about excluding animal parts from marketable comodities. Like when police burns drugs or rhinoceros horns. Would it make more sense to sell that stuff to satiate the market to prevent drug dealers profits/ save living rhinoceri? I guess so. But it would sand message that it is ok to do so . . . in the long run it hurts the cause. Applying that to our problem - we need to not think about leather as a material, no matter how long ago animal died.
Nice points Jawik!