There's more to avoiding firewood scams than just not paying in advance:
Delivery late in the evening when it's too dark to inspect the wood.
Good looking pieces on the outside that are unloaded first. When you walk away satisfied, the bad stuff gets unloaded.
Half your order gets delivered and you're promised the seller will return right away with the rest, but he wants full payment.
Wood that isn't adequately seasoned. This won't matter if you stock up a year ahead, as I do. But if this is important to you, invest in a wood moisture tester. They are good ones that cost less than $30 and that are easy to use.
Note that firewood in WA state can only be measured in cords or fractions of a cord. A description like "pickup load" is not legal.
Having said that, I have mostly found many honest and reliable people selling wood. But I also experienced one who was great for two years but fell on hard times. The third year his wood was crap. I had recommended him to a friend who was very disappointed. I felt bad about that myself.