Hi,
You may be asking two different questions. You should consult an attorney and a CPA. (I am niether!)
I don't believe there is any 'legal' requirement for interest on any sale between two individuals. There may be 'legal' restraints on how HIGH a rate of itnerest can be. There may be a 'requirement' for interest to be charged on a sale if there are other considerations involved than just the transfer of property.
IRS rules may apply on both principal (capital gains?) and interest payments (normal income?). Depreciation?
Good Luck!
Source: http://www.abusinessmentor.com