In a nut shell, if you are growing heirloom potatoes you may well have to save your own seed. If you do, be sure that the plants you save the seed potatoes or those near them did not have late "potato" blight. If that ever happens they are only good for eating. If you are growing common varieties like yukon gold etc., it is probably a better bet to buy certifies seed potatos. Don't save potatoes for seed if you can't easily recognize blight. If you replant blighted potatoes you will contaminate you garden soil for many, many years to come. Most potato growers spray their crop to keep from getting potato, blight, but without really good crop rotation some fo them still get it and then they are done planting potatoes for at least 7 years or longer. If you till or double dig you garden site you are spreading tomato, potato "stuff" around farther than where they were planted. Old timers, that were "poor" used to save seed from their potatoes for 1 or 2 years tops and then start with certified seed again, according to my grandfather.