Alp, we can buy seeds two ways (there may be more, but I don't know about them). First, we can get bags of seeds to plant fields. The seeds come from the provider in reinforced bags which are emptied into seed hoppers attached to a tractor, and off you go. They are certified by the seller as disease free. Most likely the seeds are washed to remove any traces of soil that might cause problems.
The second way is bulk seeds, which is the way we buy ours. We go to the feed store, ask for 5 lbs. of seed potatoes, or 8 oz. of turnip seed, or 1 lb. of corn seed (this is field corn, not sweet corn). The seed is weighed or measured out and put in a brown paper bag. The feed store has a sign from the seed provider that says "non-gmo, disease free seed", I've never seen a bit of soil on the seeds, but the potatoes do have a bit of soil left on them. (Remember, we are a very small farm and buy small amounts of seed. I've seen large semis ((lorries)) going to neighboring farms, loaded with seed.)
Most farmers buy seed annually. Keeping back enough for planting next year, much less storing it properly during the off season, is too labor intensive, expensive, and "iffy" to justify the small savings of "my seed vs. your seed".
Now that I've thoroughly confused everyone, perhaps a member with more knowledge will weigh in
!