Any provision of a declaration or other instrument made pursuant to this title which requires the owner of a unit to engage or employ the developer or any subsidiary or affiliate of the developer for the purpose of effecting a sale or lease of any unit is void. Any provision of any contract for the sale of any unit which requires the purchaser to engage or employ the vendor or any subsidiary or affiliate of the vendor for the purpose of effecting a sale or lease of any unit is void. The provisions of this section apply to declarations, instruments and contracts made prior to and after July 1, 1974. The provisions of this section do not apply to a condominium that is occupied and used solely for nonresidential purposes.