Just brought a car and want to sell it without transferring?

I brought a car last Saturday I've already spent $400 in repairs and basically I dont want it anymore. The lady signed it over but I didn't fill out the back because we wasn't sure who's name we was going to register it in. So basically I have the title signed over from the registered owner. Ppl get suspicious because its not in my name but to do that i I would have to pay taxes, smog it, and get insurance just to transfer it so another like $400 just to put it in my name to sell it. Do i have to register it or is just selling it without my info on the title from the registered owner who signed it over straight to a new buyer ok?