Can I get into trouble for this?

Long story made short:

Friend: josh
Honda guy: hector

My friends car broke down but I know of a guy who sells cheap Honda parts so I told my friend josh that I know of a guy who sells cheap parts and that he would charge $100 for the part he needs. I ask my friend the year and engine of the car and he told me it was a 2005 Honda Civic. Josh gives me $100 and I go buy the part from hector. I buy the part and give it to my friend. It turns out his civic is actually a 2008 and it's a completely different engine. I go back to hector and he tells me there's no refunds or exchanges. We have to keep the part. My friend josh gets pissed off and started telling me that I'm responsible now for the $100 that we lost. He wants me to pay him the $100. He said he is bringing the cops to my house today. I don't see how I did anything wrong. I also don't even have any money to give him either.

Can I get into trouble?

What exactly does your friend think cops can do? The police department does not handle things like this and would not come to your home just for this. If he wants his money back he has to take you to small claims court. But, ask yourself, is it worth losing a friendship over $100? Offer to pay $50. Then next time do not accept money from others and they can do the job of going themselves. You can tag along if needed.

I would hope not, just tell the cop what happened if they ask you

First, the police will not get involved and they will not come to your house. Second, not your fault, nothing illegal was done

As long as you provide Josh with the (wrong) part that he paid for, you fulfilled your obligation to him and can't be held accountable. This is an issue of civil law, not criminal law and police only enforce criminal law so they won't respond to Josh's demand for them to help unless he lies and tells them you forcefully stole the money from him.

You bought the part that your friend said he needed. The police have better things to do.

1. Being the middle man in a transaction for someone else is terrible idea made much worse for used parts with no warrantee. Why would anybody do that? 2. I thought Josh was your friend? Apparently he is not because he sees no loss in taking the parts back. Since Josh is being a total jerk, you are screwed because of extremely bad judgement.

It's going to be your word against your friend's word, and the Police will believe one of you. I have no way of knowing which one of you will be believed.

I also don't believe the Police are coming to your house.