The scam is: I want to buy your truck I can have my brother in law to pay for it, he is in your town closing up a deal for me. Problem is that my customer gave him a check for ****** many dollars, can you deposit it, wait for it to clear, then give him your personal check for the balance and keep $500 for the trouble, I trust you. Then just give him the truck and we are all good.
The bank will clear the check because you are a good customer, but they find out the check is bogus and take the money out of your account (yes, its legal) and you are screwed.
If you feel any hint of discomfort now you will feel more discomfort when you find out what happened.