Can anyone buy ETH for me if I pay you through a payment processor such as stripe? I will give you the american express transaction fee and a 3% fee on top of that. I am choosing to do this with american express since it gives me safety in that I can cancel the credit card transaction if the ETH is not sent through to my account which I will monitor for your transaction on the ethstats
Usually people will not do this because the risk is too high for the seller. They could give you the ETH, then you could turn around and charge back the transfer. They would be out the ETH. Usually you would need to use a trusted escrow service.