A marking knife is used for marking where you will be cutting with your saws. For getting into tight spots (like dovetails) and making very accurate lines (which is vital for tight fitting joints) you need just the right marking knife. You would think that any old knife would work, but you would be wrong. Years ago I purchased several that didn’t work well.
Thickness planer. A thickness planer is used to fix surfaces of imperfect wood. If you are picking up less than perfect wood from the lumberyard, wood with some surface defects, a planer will let you fix that wood in the quickest time. A thickness planer pays for itself quick as it allows you to buy cheaper wood that doesn’t have perfect surfaces and fix it yourself. Be careful though. You loose wood during planing. So the remaining amount of wood still has to be worth the price you paid for it.