but of course that raises the issue of you having to trust me. Ugh, I wish this were easier!
This is why you use a trusted 3rd party, or Escrow.
You send the cash to the escrow, he sends the BTC to the escrow. You get the picture.
Trust is based on history, so we need trustworthy people to run escrow services and gather good reviews and who are happy to receive funds and exchange BTC. All for a fee of course.