There's a global bonus and a local bonus. The local bonus didn't apply to merchants after I'd upgraded it to a Master and then HQ level building, but the global applies everywhere. It might be a bug, but in my Byzantine game I build my merchants in another city that only had a guild. I'd get Legal Nouse there, plus Financial Training and Merchant's Guild Apprentice that way. If I built them in Constantinople, I wouldn't get the Legal Nouse trait.