Seed potatoes are so inexpensive, it's not worth using old potatoes (though I have done that many times).

I don't know where you live, but around here supermarkets will soon be selling 5lb bags of seed potatoes at comically low prices. It is ridiculously cheap to grow some staples: I can buy 100 onion sets for $2.00/bag at my local "pricy" feed store. Enough corn to plant an acre costs about $5, etc...