I mentioned what I was basing it on. It is my understanding that the documents McDonald used have been released and so far no one has found anything. Perhaps someday the full truth will be known and I may be proven wrong. Perhaps, as Steve suggests, Shaw had a closer role with the CIA wheather he was paid or not. But the bottom line, as you point out, is that there is no evidence that Shaw had anything to do with the assassination.
If the Carpenter characterization is accurate then Shaw was directed or instructed by the CIA to seek out certain areas BEFORE he went abroad. That is, he didn't come back and answer questions; he was instructed to seek out certain areas/subjects ahead of time. And Carpenter documents that Shaw traveled abroad extensively while promoting the Trade Mart (it was, after all, called the "International Trade Mart"). Both to Europe and especially Latin America.
That's a closer relationship than that of the thousands of Americans who were briefed by DCS about what they may have seen or heard when they returned from abroad. I was once interviewed by the FBI in the 1980s about some sort of Polish intelligence operation (I think). It was a pro forma interview about a particular person (she was Polish and had been a co-worker of mine). I knew nothing at all about it. If I had been asked by the FBI before this to look for suspicious behavior by that person at the time we worked together that would be a completely different relationship with them.
BTW, I never really knew what the heck they were looking at <g>. It could have been simple tax fraud or something mundane. But why would the FBI ask me about something like that? Anyway, it's always a good Thanksgiving dinner story; although I'm pretty sure my relatives are sick of hearing it.