It's weighted when the market isn't common. Common means access to the same raw material, Labour, tariff etc. To be fair, if it isn't really common now. Taxes, interest rates, other overheads are certainly not common, but to make it as common as we can, two very important factors are;

Access to raw material means we all pay the same tariff for, for instance, US steel or Chinese widgets. Labour means workers can move around the zone seeking better employment.

They are the sacred cows and as Norway and others have found, to trade with The EU, you either get into tariff or follow their rules. Turkey uses the former, Western non EU countries follow the latter.

Just to address the hilarious post above you.. If I want to read that illogical paranoid crap, I can always read the UKIP leaflets or your favourite tabloids. We do not have unfettered immigration. In fact, quite the opposite. You really need help Akenaton. You see the manifesto of the party you tell us you are a member of as a liberal plot, think that equality means unfettered immigration and that normal people think down at your irrational level.

I didn't think Sturgeon was so desperate for votes....