When this is the case with certain vendors it's usually because they only want established customer to limit people try to pull a fast one. Once ypu have a decent buy history they will usually grant access via there password if you message them. Hope this helps :)

I use coinbase (https://www.coinbase.com/) then transfer the amount into it, then covert it from £/$ to btc, it takes a tiny percentage for the exchange but once it's transferred ***important*** download electrum (https://electrum.org/#home) and create a wallet. use this wallet (electrum) as your main wallet and transfer your btc from coinbase to this wallet then spend from this wallet with the address provided. message me if you have any problems.