You probably didn't see my edit, which is pretty relevant :o)
US law requires knowledge of the goods being stolen, not mere belief. You don't have to convince the jury you didn't know, the prosecution has to convince the jury you did. That's a very high bar to meet, and while it could go to trial it almost certainly wouldn't, not unless they had solid evidence of his knowledge (eg, if he said they were stolen on his stream).
The fact that they chose to go through the Pinkertons more likely points to the fact that they knew they wouldn't have charges thrown about if they involved the police. The buyer would have been less likely to cooperate.