MichaelB. said "You need to have the Transfer of Ownership Document (which is on the back of the registration card), signed by the previous registered owner. If the person you are buying the car from has the TOD from when he/she bought it, they just need to go to MPI and register the transfer to them (which may have a $15 fee attached). They can then get a new TOD and transfer it to you.
Be aware that if they bought the car and never registered it, there may be sales tax owing, which will have to be paid by them before MPI will pony up the transfer documents. There will also be sales tax owing again when you go to register the vehicle.
If the car has never been registered since MPI brought in the TOD regulation (around 2005) getting a TOD will be a headache. It can be done, but you need to decide whether the car is actually worth the a$$ache. Better for the seller to do the legwork and just hand you the signed TOD. "
Ah yes the old PST ripoff where you pay tax over and over again every time the same vehicle sells.
Also remember if you sold a vehicle within the last 6 months and replaced with a new vehicle you can claim the tax back or portion of it depending on what you sold yours for and bought the new one for. You will have to ask for the tax form obviously as MPI or any of the brokers won't tell you this otherwise. Oh and also the Government with drag their feet for anywhere from 3 - 6 months to process it and issue you a refund cheque. Have fun!