My main thought on this idea is that you have no guarantee of getting raspberry plants with worthwhile fruit if you grow from seed. Not to mention the time you'll need to grow the plants to fruiting size. It is good practice to weed out any seedlings from a raspberry patch for this very reason. If you leave them you run the risk of ending up with a stand of very inferior small, bobbly berries. It is much better to buy a named variety of known quality, even if only a small number of canes to start with. They will soon increase and you can propagate more from them and be sure to have fruit worth eating. Would the people at the farmers' market sell you some canes? Or at least tell you the name of the variety they're selling so you can buy some elsewhere?