Do not discuss doing commerce. I have had several experiences taking parts for my racing engines to specific engine service companies with great reputations, including proper documentation. When a border agent decides to be a dick, be prepared for problems and watch your words, attitude, etc.
My personal items have nothing to do with commerce. Racing is considered a business in the U.S. I just lost money to racing a car, basically a donation to the track for the privilege to use a race track. For me going fast, legally and safely, was all I cared about. So bringing a guitar to ship, could open a can of worms; or not. The border agent is there to justify your entry into the country. I also don't want to make problems with my NEXUS. They have threatened to take it away, for not liking hearing the truth.