Personally, I have sold basses to buyers in Germany, Denmark, Portugal, France, Monaco, Italy, Belgium, and The Netherlands.
Each one went without any problems (although the delivery to rural Portugal took a couple of days longer than expected). Because of the the EU membership, it was a really straightforward process, with no cross border problems (even the sale to Monaco had no issues).
If Brexit actually happens, it'll be back to the old days of import / export taxes and tarrifs being applied to such sales.