it is okay to exchange BTC for ONE THING or THE OTHER THING but permission is needed if you are only or mostly buying and selling BTC for money.
What else are Turkish phone shops doing then "only or mostly buying and selling Vodafone-codes for money"?
Because these things are not a currency, they are means to an end - vouchers.
These are specifically exempt from regulation.
Please do not make me argue German Law with you, just read it yourself.