I agree, well saidMy opinion- for what it's worth.
"Thank you for your custom" is something a seller would say to a customer anywhere that English is the first language. It's in the .com dictionary and i've heard it on American TV programmes, so just because one or two US sellers are unfamiliar with the term, doesn't mean ebid should change it.
Thank you for your business is similar- but does not mean exactly the same - and is not something I would say to one of my customers.