If the retailer is based in the UK then they would have to abide by UK legislation. Once the sale is made the importation into another country is not their responsibility. Hence you can buy a RIF from Taiwangun but when it gets to UK Customs you need a defence to bring it in.
So if someone in oh, I don't know...Italy wanted to buy from a UK store, that store would have to be satisfied that they weren't breaking UK law but the aforementioned Italian chap would have to ensure any information required to get the RIF into Italy was provided otherwise the Italian Customs chaps could seize it.
^yep, also need to bear in mind certain countries have some strange restrictions, like germany and their 0.5j full auto.
shops should be fine with it, my local does a lot of semi-auto only conversions for german customers, but bear in mind that as mentioned if he tries to bring it back in without a defence then he's not gonna have much luck.