Noah, the term "trading post" is often overused in my opinion. Lots of times these were simple tavern/inn sites where folks stopped for the night (or two) and paid to lodge their horses etc. It was simply an older version of a bed and breakfast. Other's were just places in the field where farm hands were paid, or it could have been an old produce stand - found a few of those. Now, I have found that some "taverns" were more along the lines of a place to eat and dine - like a bar, while others were just the former. If the site is like the latter, then more old coins are likely. So when I find a tavern, I never assume there's gonna be handfuls of coins lying around everywhere, but there will likely be some stuff there, especially if it was well patronized. In terms of finding them, by far my most tried and true method is to read through old town history books (some online, some through libraries) and then cross reference names/locations with modern day maps. I've only had one instance ever where a property owner told me about a site. That one did prove amazing but in general those are the exception. So put on your reading glasses first, then turn to the maps. If you can find ones along major highway routes (even if they're currently in use) then those typically produce better than out of the way places. I've had success finding a few like this.
John