I am not a lawyer, but according to the Nevada DMV's site:
Selling to a Private Party:
Provide a proper title to the buyer
Complete and keep a Bill of Sale (Form RD-104)
Keep your license plates
Did you do all three? You mentioned the original bill of sale...what happened to it?
The page continues:
The Bill of Sale is your proof that you sold the vehicle. This proof is particularly important in case the vehicle is abandoned at a later date. The buyer does not need a Bill of Sale to register the vehicle if you have provided a properly signed-off title.
So it sounds like the BOS is your only real proof that this guy got the car impounded and not you. Did he register the vehicle in his name before the impoundment? Finally, was the new BOS that you had him sign dated for the time of the original sale? Or was the date on it AFTER the impoundment?
It seems like we can take the following to be true: Nevada does not require you to submit any paperwork when you sell a vehicle, as the only thing someone needs to register the vehicle in their name is the signed off title, or a BOS if that is not available. A BOS is proof that you no longer own the car. In other words, according to the DMV website, you are under no obligation to do a damn thing. So it sounds like you should be fine as long as you have a BOS that is dated prior to the impoundment. If you DON'T have that, then you may be out of luck unless this guy is willing to sign an affidavit saying that he bought the car.
If I were you I would look up the relevant statutes in the nevada vehicle code, photocopy them and your bill of sale and send it to both the towing company and the collections agency along with a letter explaining why you are not responsible for these costs. If they don't give in, then you should spend the $100 to sit down with an attorney for an hour in your area. He or she will either explain what you should do next or will tell you how much it will cost you to make it go away.
Finally, if it ends up on your credit report you should dispute the account and add a comment to your report.