That's actually one of the more interesting little mentioned aspects of this election. Trump and Sanders don't disagree about everything

They're both against NAFTA and that new Pacific trade agreement and have both criticized companies for exporting jobs to Mexico.
They're both fairly dovish on foreign policy and against our past military adventurism and nation building.
They both want to make friendly with Russia. Trump questions NATO's roll since we're not in danger of a European Soviet invasion anymore, and Sanders called for a new NATO that includes Russia.
Obviously there is a lot they disagree about, but it's interesting how popular anti-war and anti-(un)fair trade is with both Republicans.