Is that the US customs paperwork or the UK customs paperwork?
Also, the definition can be different even within the same statute/regulation - that's something people without legal training often miss. Just b/c a gov't says the def'n is "x" somewhere doesn't mean it is "x" everywhere. It happens within the CPSA legislation, in fact - people have been confused by what certain categories mean & have erroneously thought their items were exempt from everything when they were only exempt from a specific requirement.
I know from my work in family law - what is a spouse, a parent, a dependant etc. is remarkably inconsistent even within the same law sometimes.
Anyway, even if the interps I have read from lawyers in this field are wrong, putting it all on the buyer would mean saying "It is illegal for me to sell this in the US - You will take all responsibility for the consequences" in your policies & listings. I'm not sure that looks all that great for your business :)
I also bet that if people are doing this, Americans will be likely to buy from Americans first, especially with their economy. Just another thought