If I saved up my money and bought a tractor
What if I instead offered to plow their fields for them instead
You assume it is necessary to use money to buy a tractor for yourself, you assume "their fields" are owned by "them." What if the state provided you with a tractor and the land? Or even if money were involved, what if the state provided you with the money to buy the tractor and the land?
There would be laws to allow you to hire other people to use the tractor and farm the land, but by law, the surplus of their labor (whatever they planted, farm, sell at market) would belong to them, likewise the surplus of your own labor would belong to you. You could use the surplus (money, goods, what have you) to trade with anyone else.