Well this is a pretty decent place to start for our first Resale-Flash, it's not a massive under average pricing but its a decent place to start. This Western Digital SATA Hard Drive WD6400AAKS, it's SATA 3.0GB/s, 7200RPM, and has a 16gb Cache to make it nice and fast. Newegg has this WD 640gb hard drive selling currently for $69.99 which is the lowest price currently I have found the drive for. Beating out everyone else on Google Checkout search.
The average selling price ranges from 74$ to 105$, that means at 70$ including the free shipping that NewEgg is offering on the item you can make between 4$ and 35$ reselling this item. With the low cost even picking up 1 or 2 of them for resale seems to be a good idea, maybe order one for yourself and expand your own computer while your at it.
Google Checkout Search - ~70$ - ~105$
EBay - ~73$ - ~99$
NewEgg (RESALE FLASH) - $69.99 Free Shipping