I've been selling firewood for 19 years now and sell more wood each and every year. There is no one answer with advertising as you have to change as the times change. First couple years I actually paid a small amount to advertise in the local paper. Then craigslist was the answer for me before it even became popular. A problem with CL though is too much riff raff with guys selling cheap green short loads but advertising as full and seasoned. Next thing you know people are asking why you are so expensive compared to them (easy answer is buy a cord from both and find out for yourself but its really not worth the hassle) Recently, I have used community facebook pages to either put an ad up on or respond to a person looking to buy wood. This exploded on me in a good way this past fall. I had a waiting list just to deliver wood 2 to 3 weeks long and people were still lining up. When someone put up a new post asking about firewood, half a dozen people who recommend me before I even logged on to see the thread.
You have to start with quality, it has to be clean, seasoned, and should all be cut to the same length (16" works best for my market). If you are selling oak, it better all be oak, not some sort of hardwood mix. If you sell hardwood mix, no pine, no exceptions. Be picky about pieces that have rot or are punky. Customers don't want to pay top dollar for that. Get to know your market for split sizes. My people like it split fairly small so even though larger splits decrease my time, they will also decrease my customer base. I have a lot of women and older folks who don't like handling the big splits. They aren't going to be happy if they have to resplit pieces. Show up on time for delivery. I can't stress this one enough. If I'm going to miss my delivery time by more than 10 minutes, I'm calling or texting them to let them know. Deliver full cords or fractions of, never short someone.
So basically, treat your customers like you would treat your mom if you were bringing her firewood and you will have a great customer base before you know it.