Dutch etymology says koopman comes from köpink, copunoa [9e-10e century) from köpon - 'buy'. with links to German Kaufman, kopen 'buy' was borrowed from Latin 'caupõ' meaning a small dealer, also innkeeper, sjacheraar. Possibly just like Greek 'kápelõs `id.' coming from a non Indo-European language.

I see no mentioning of 'head'. it 's all related to buying.
Yes, koopman is still a current word also current is colporteur which is a chique word for travelling sales man.

smile Cole Porter maybe started as a singer going from door to door? ( I like his songs)

Yes I read yesterday's word but got distracted by the wisdom of the day.