For privacy- sell it without the hard drive. The RAM is fine as it is volatile meaning once the power is interrupted or turned off the contents are erased. Adjust the sale price for the lack of hard drive also. Quite a bit of us on this forum are pretty seasoned when it comes to tech so we have spare parts lying around (hard drives, RAM, cables, etc.) so no hard drive won't necessarily be a problem.
If you absolutely want to sell the Thinkcentre along with the hard drive- download DBAN
. Both programs provide burnable ISO files that you can make a bootable CD with. These programs should be enough to rid your hard drive of it's contents.
Try selling it here
Put an ad in the marketplace and see how it goes. You could also part out the desktop if it came down to it. Good luck