I highly recommend you do not take the $100 for these reasons:
Regardless if she has paid you and that you've signed over the title, the law states you are still liable if the new owner gets into an accident unless she is driving the car under her OWN registration.
Once you've signed over the title, your registration (unless it's expired) and insurance (unless you cancelled the insurance already) is STILL VALID. If she did nothing but pay you, and you sign over the title, and she drives off with it with no plates, it would still be your responsibility if anything happened to her.
Until the buyer obtains a "Temporary Registration" (similar to what you get when you purchase a car from a dealership, those annoying paper tags that are taped to the rear window), the seller can still be held liable.
The buyer must obtain the "Temporary Registration" from NJ DMV (or MVC... whatever the hell it is now)...
http://www.state.nj.us/mvc/cit_regis...emp_in_nj.html
It is a $5 fee.
Of course, you could take the extra $100, but you risk the buyer getting into an accident, and you being held responsible. EVEN if the buyer purchases insurance with the VIN# and all, you can still be held liable.
So, it's a risk... I would make the buyer drive a few extra miles to pick up the car and get the temporary registration.
(I speak from experience about this whole matter; when I bought my motorcycle in New York, I had to go through the whole procedure of getting temporary tags... it was not a pleasant ordeal. I purchased the bike from a lawyer, and he didn't want the liability. At the time, he showed me all the driving statutes and how the Seller could be held liable.)
Just my 2 cents...
~Joe