Hello Guys , Maybe im just stupid or i dont know how to pay using Bitcoin to a website

So i have 850 USD in bitcoin on GDAX.US , looking to buy something from a local website but thats doesnt accept from GDAX , it require application on phone to scan code and thats i can only with bitcoin wallet from android market

i can pay someone to help me if there is someone that know what about ... or i trade the GDAX 850 USD BTC for ... 750 USD BTC on another Wallet



looking to trade only with reputated users , il go First in that case