Sell firewood. This is the simplest way to make the most out of your log splitter. While this can be a very good way to make money, don't go overboard at first. It does carry some risk that you might not get back the money you invest in buying unsplit firewood. Start with a small amount, post signs around town, and hope that you can get the price you're asking.
Advertise online if you're not moving your wood fast enough. Craigslist is a great resource for people trying to unload their stuff and make some money in the process. There are probably plenty of people in your town looking for split firewood, but maybe they just didn't see the signs you've put up. Advertising online can really help you move split wood, and it may help you find repeat customers.
Offer to split firewood for a fee. This carries much less risk, but also limits your market. In this case, your customers need to have the firewood themselves, be able to move it to your splitter, wait for you to split, and be willing to haul it back. That's a lot to ask from somebody who is paying for a service. Alternately, you can bring them split logs, take their unsplit logs and a monetary kicker for your services.
Split wood for free. If your neighbors have a stack of firewood that needs splitting, and you have a splitter, it would be very kind of you to split their firewood for free. They will definitely appreciate the kindness, and they will think very highly of you. While you shouldn't expect them to, they may decide to return your kindness in some way later.
Never move firewood across state lines. It's not your fault if the wood you get has some kind of fungus or parasite beyond your control. But it's your responsibility to try to contain that problem. If you move firewood long distances, these fungi, diseases, and parasites can move into more forests and infect more trees. The trees in your area might have built up a defense to the fungus, but trees in other areas might not have any defense to the new threat. Moving firewood can destroy forests and upset ecosystems. It's just plain irresponsible, and usually illegal.