Trying to verify and sign a transaction

[andrewpttsbrgh]

I made an online transaction using bitcoin. I sent the coin and was told the product was out of stock. I was given a what I'm told is a signed transaction, told to verify it in coinb.in then sign with the key I was provided. I put the signed transaction into 'verify' in coinb.in hit submit and I am given and input and 2 outputs. I go to sign, use the provided key and I get a message saying there is a problem with one or more of my inputs. I'm using coinb.in because I was told to, not that I know how to use it. Any help would be appreciated.