Negative trust and rep can be overcome by doing the right thing so people know that you are a good person. By keeping his money and not sending him the BTC you are proving that you are not a good person and he is right about you. If you really are a good person you would do the right thing and make the deal as promised. Good people don't worry about not making money on accounts. People like you kill the whole online community.
First of all you are SO SO SO wrong when you say neg rep can be reversed. The stuff that has happened has happened and simply cannot be reversed. That is a fact.
Integrity is hard to keep up when you are poor mate. Good people cannot always be good in tough times. I'm sure you won't either if you were in a really tough situation like I am in now.
Like I said integrity is hard to keep up when you are poor.