Your liability depends. If the Contractor isn't insured, you could be VERY liable. If they are, get WRITTEN PROOF (a copy of the insurance) that they are insured.
As far as 'selling' the shingles, are you talking about USED shingles??! If so, you'd have to check w/your municipality. IF you aren't allowed to sell them, see if you can GIVE them away! Then check w/the State EPA. Then check w/the Federal EPA. Otherwise, you or your contractor needs to have them properly disposed. They are considered Hazardous Waste.
As far as being able to BUY the shingles, Home Depot sells them. They're about $1 each if I'm not mistaken. So, in a way, it's not illegal to sell them. Maybe you need to be 'licensed' or something to do so. If you're looking to 'change' your siding, that's another matter. (Let me know more about what's up if you are.)