Nikon D5000 /D7000 portrait?

I need to upgrade my Portrait with Nikon D5000 such as using a prime lens 50mm 1.8.
Do you think i have to use better body such as Nikon D7000 which has better sensor.
Nikon D7000 now cost about $1100 but in ebay with positive rating 99.1% now cost $900 but i'm affraid of the increasing No.of defects and loss of reliability by buying from ebay.

Added (1). I need to get sharper images with better contrast.

What exactly do you need to update and why?

The D5000 is completely capable of shooting portraits using a 50 mm lens.

What you need to do to enhance your images is learn how to light for shooting portraiture. Shooting portraits is all about lighting.