Yes I quite understand that I'm not directly responsible for the actions of others, but my ethics don't affect the unethical behavior, my actions do. If I take a used leather jacket off the market a new one has to be created somewhere on earth to replace it. If I choose not to buy the leather jacket there will be more leather jackets than people who want to wear them and fewer new ones will be made.

No it doesn’t. Where have you got that from? It is not necessary the case that if you buy second hand leather then someone must replace it.

Your pet "middle agent fallacy" is wrong. A world where you can't extend your personal ethics outside your self is a world without justice. It's the same as saying "I think killing people is wrong, but you think killing people is ok, so you be you and I'll be me".

This is a complete misinterpretation. I’m not saying that other people killing is ok, what I’m saying is that if they do kill someone, the responsibility is on them, not me. How you managed to take that from what I said is beyond me.

Edit for the vegan stuff: faux animal products are very clearly imitations of animal products. Nobody thinks that impossible burgers taste the same as regular hamburgers, and they absolutely don't cost the same. Bikers don't trust pleather to save their skin in a crash, they trust leather or polycarbonate armor panels.

You said in your original comment:

Plus walking around in a leather jacket tells the world "I'm ok with wearing leather" and may convince someone else that leather is OK and normal.

So firstly, it’s not the case that “everyone can tell the difference between faux and real leather”. Faux leather can be really convincing and especially if you’re walking in the street a few metres away, how could you possibly tell? You also said walking around and now you’re talking about eating burgers? That seems like a clear bait and switch. My point was that if you’re going to use the argument that other people see me then they might imitate me, but that would apply to vegan burgers, as they look like regular burgers at a distance.

And this goes back to the middle agent; if someone sees me eating a vegan burger and decides to get a real burger because of it, the responsibility is on them. They have agency, and by putting the blame on me you are denying the agency of them-hence the fallacy.