
Your shop may be selling local farm tatties but you can be sure they are plentiful maincrop ones (that's why they are cheap - simple supply and demand pointy head economics) and they will have been sprayed with fungicide as a seed tattie, drenched in a pre emergance weedkiller at planting time and then sprayed for blight every 6 weeks throughout the growing season and then have the shaws/haulms burnt off with acid before harvesting (wander by a tattie field in September and you will see the scull and crossbones warning sign on the field gate!). Commercial tatties varieties are selected for disease resistance & bulk/weight, certainly not flavour!
Seed might be expensive but as Stoney says, you can choose the best varieties, be they organic, local or heritage. You can save your own seed too - just hold back the smaller tatties and keep the frost free until early spring and then bring into the light and chit in the normal manner. Commercial tattie farmers will buy in fresh seed every 4 or 5 years but I know a local guy who has had his own seed on the go for ever.