It came with either Snow Leopard or Lion. I have a Mac Mini mid-2011 which came with Lion pre-installed.
If your system came with Snow Leopard, you need to boot from the DVD install disk, erase the Macintosh HD and install Snow Leopard.
If it came with Lion pre-installed, make sure you have an internet connection. Restart it by holding/pressing the COMMAND+R+OPTION keys until you see a white screen and spinning globe. This is Internet Recovery. You will get the OS X Utilities screen where you can erase your Macintosh HD via Disk Utilities and then reinstall Lion. After Lion has downloaded and installed and you see the beginning of configuring Lion, you can shutdown the Mac Mini and it's ready to sell!
Once you erase the Macintosh HD everything will be gone. You may have to find out from Apple (1-800-MY-APPLE) how to allow your new buyer to download the iLife apps (iMovie, Garage Band, iPhoto) after they receive the Mac Mini and configure it. Right now, those apps are linked to you via your Apple ID.