Yup, I bought my legend from private owner in Rochester, NY while I'm still in RI.
Usually you have to bring your own license plate, or you can apply temporary plate from your DMV. But, if the seller is kind enough, he/she will let you drive the car home with his plate. My seller allow me to borrow his registration, so I'm all set with that.
And, before you register your car, usually you have to have the car VIN# to be inspected by your local DMV to make sure that the car is legit, not stolen, etc. And you can register the car later on.
And now, for the tax. In RI, if you buy it from the private owner, the tax is assessed from the book value of the car, so eventhough you have a bill of sales for 1 dollar (cheating), the tax is still the book value, but different state might be different. In RI, you have to have insurance before you register the car, but it's pretty easy. In MA, it's different though, you need to get insurance, and the insurance company will send you the registration form with stamped verification that you had insurance from them already.
And, as soon as you get insurance, of course you need to get the car inspected for safety and emission (if any). And, usually your car need to be inspected too for the insurance company to prove that your car is in good condition, so you cannot cheat on them later on.
I think that's about it.