I'm quite certain "to trade for" is "to travel".
Your interpretation, Joe, of the chorus is also quite right.
So that only leaves "sae there", in the final verse.
I was intrigued by 'skeely skipper' in the first verse. I see skeely means skilful in Scots vernacular. I should have remembered 'skeely skipper' from the Ballad of Sir Patrick Spens. But it is 50years since I learnt it at school!