Urm, urm, why not be proactive and simply put a note in your advert when you're selling something saying you're outiside U.K. or simply check if this item you must have is in the U.K. or elsewhere. If it's elsewhere, you'll have to pay the VAT, the incoming taxes and the adminstrative fee.
There is no need for such a caution, just a bit of common sense.
That said, a lot of private people are just not selling anymore to the U.K.