Not sure what your exact error is but sending BTC direct from coinbase to the sellers wallet is generally not a good idea, better off getting yourself a BTC wallet to send the coins to once you've bought them from coinbase, then send them from your wallet to the seller!
Exodus is a really good wallet i know lots of people using it, really simple and user friendly!
Exodus is a really good wallet i know lots of people using it, really simple and user friendly!