Your model came with a set of software restore discs. You need to use those to erase the drive and then install a fresh copy of the original OS X version that came with the computer. You cannot use the retail copy of Snow Leopard. You will need the original discs. If you don't have them, they can be purchased from Apple:
Apple Store Customer Service at 1-800-676-2775 or visit online Help for more information.
To contact product and tech support: Contacting Apple for support and service - this includes
international calling numbers..
You cannot sell or give away your Mini with any version of Lion or later installed. Those versions are all permanently tied to your Apple ID and cannot be transferred.