If you are looking for paperwork/a paper trail for any transaction then your best bet would probably be coinbase - I believe they are serving most of the EU now. You would actually get the market price and your trade would be able to be better documented then most likely anyone on here can do.
If coinbase doesn't work then you would probably want to use a major exchange.
Thanks for the tip.
I will contact Coinbase and ask if they can issue an invoice. All I need is a pretty paper with a company name, amount bought and sold, i.e. an invoice / aka paper-trail.
From what I can tell, Coinbase does not offer that on their site.
I would not expect other exchanges to invoice, as they don't sell coins per se, rather exchange between users.
