I have been selling firewood since about 1982. Most of the time I refer it as a spare time business although it was most of my earnings occasionaly. IMO it is best treated like a spare time business in area's like mine. If your close to a metro area where firewood sells for premium prices I reckon you could rely on it more. I can only think of 1 person I know or have dealt with who has done it longer and has made it a big business. He is up near the metro areas and gets premium prices. He also grain farms quite a bit so it I would guess is about half his business. I believe he usually has at least 1 person helping him, buys quite a bit split and stacked at his place,some unsplit and some tree trimmers drop him off wood free last I knew.
Usually there is quite of bit of wood in hardwood tops.Might pick some up reasonable and find a partner or hire help, cut them to 16" lengths, bring it to the wood yard and split it unless you can sell it direct. If you buy unsplit wood without paying to much after you split it you usually gain some volume so i'd take advantage of that if you can.
Its easy to get a lot of man hours in a load of wood by the time the customer has it and pays you,+ expenses ( equipment, maintnence, insurances, vehicle cost, oil and gas, and there is bookwork). Hope you don't pay to much for the logs, unless you get a very premium price for firewood you could easily get down to working for pennies an hour, all things figured. I have seen a lot of people sell wood anywhere from a season to several years and quit. For me doing a lot manually I've been content with my less then modest earnings most the time.
I don't have a family to support, rent and a lot of big expenses like your average US citizen though . I see some tree services sell wood in the winter their off season which looks to me more of a reliable year round income.
If you need more clientel, in IN there is a home heating assistence program through Human Services. Some people take firewood rather then propane , electric or oil. If you register with the program as a vendor you can supply wood to these people and get paid through the program.