Well, your 22 years with US Customs certainly overshadows my meager knowledge on the subject. However, I would like to point out again that we're talking about a guitar being sold in the UK, to another person, in Germany, neither of which has anything to do with US Customs regulations.
More importantly however, under those conditions it isn't being "returned" anywhere, (especially to the United States), it's simply being sold and shipped from one country to another.
I know what you mean about regulations "changing" every few years...after retiring from the USPS in 2004 with 42 years in the barrel, I've noticed many Postal regulations changing, just in my seven years of freedom!
David