Janey's advice, as always is spot on. I'd only add that $150CAD seems very steep for that particular machine. We haven't seen your exact machine, but if it were "mint", it wouldn't be partially seized.
It appears the Sterling is a run-of-the-mill badged treadle machine. Not a thing wrong with that, but there's apparently nothing special about it to command a premium. Here in the northern US I can find functioning treadles for under $50US almost any day. I don't know the situation where you are, but I'd at least take a look around your classifieds to be sure you can't satisfy the urge more economically.
That said, I'll admit I once spent $200US for a cabinet ALONE and drove 175 miles round trip to get it. Sometimes money is a secondary consideration.