The benefit is that you know about the house and should be aware of anything wrong with it.
Also, your parents may be exempt from certain costs of selling the home since it is to a relative.
Finally, everyone will save on the costs of real estate commissions, although you will probably want to pay a realtor or attorney a fee to draw up the paperwork.
The only real negative is that the lender is going to scrutinize things a little closer since this is not an Arm's-length transaction. It doesnt mean you wont be able to get a loan - just means you might have to jump through a couple extra hoops.
If this is a "bailout" situation where the parents are facing foreclosure or something then you probably wouldn't be able to get a mortgage
Hope this helps!