Any site Belgium that uses BTC to buy gold?

So i went to rsgoldbank to buy gold @Cas Site,

I spoke about selling bulk rs3 to them and they said they can buy 8b for btc

For some reason they wanted me to show them my ID or they werent allowed to send me btc?

Anyone else have this problem or know why they were like this? BTC is an anonymous payment method i don't see why they need my ID for any reason they wouldnt even know if it was my name linked to my btc account lol