Establishing trust isn't overnight and shouldn't be. It takes time and I think you need a foundation to build on. There is no exactness to it either. I have had people that I almost immediately trust and bond with and others, for however long I know them, never get there. It is not that I don't trust them (for lack of a better term) but, there isn't 100% given over to them.
Parents build a foundation of love and safety for us with respect, communication, truth, and honor. The online world has ripped down so much that it becomes hard to trust anyone, especially if you have been burnt once or even twice... but especially the first time. Of course, over time you do, reluctantly at first and then, a little bit more.
A good subject to toss about. I wonder if scammers ever do.
There's no formula to apply to giving trust, it just happens and sometimes quickly as you say. Women are better at men with the whole intuition thing, but men do it too and that helps a lot. We just feel a certain way and with a little experience here and there, trust happens. We need to apply caution though also, in my opinion.
I agree it's a decent topic to discourse on, broad in scope, and no, I think scammers don't consider it. They probably don't even stop to consider their actions as wrong, greed, ego and hubris tend to blind a person.