If a tenant or an owner-occupier decrofts a site, for example a croft house site and garden ground or a building plot, and then buys it and sells it on, the decrofting takes it outwith the ambit of the new legislation as it is no longer subject to Crofting Tenure?

Yes, once the land is removed from crofting tenure through decrofting then the provisions of the Crofting Acts would no longer apply.