1. Is it mandatory having a concession / certificate / training or any other verification which permits the production & sale of handmade ceramics from clay, or is it a free trade?

Not that I am aware of, I have never heard of such.

2. Which regulations have to be observed in general with regard to the manufacture and trade of ceramic products in Sweden?

As far as I know there are no regulations. It is assumed that you make safe ceramics and not trying to poison you customers with your novel vanadium/lead/barium glaze. Obviously should you do something like that you would be held responsible in court.

3. Is there an association/union of ceramists or something similar that I could contact regarding specific questions?

Non that I am aware of, I have not looked for one so I wouldn't know.

4. When using glazes and coatings on ceramics, especially for tableware or products that are supposed to get in touch with foods: Do the glazes and coatings used have to be certified by a test center (laboratory) on substantiate harmlessness? If so, does that only apply to food-safe tableware or does it also apply to decorative items? Or is it only essential to have the manufacturer's glaze data sheets, which shows, that the food-safety is assured if you use the glazes designated for this purpose?

To my knowledge there is no testing requirement. Again it is assumed that you know what you are doing. If you glaze a plate with something you bought that clearly states "this is not a food safe glaze" then it is on you. The retailer will not be held liable. Personally I am making all my own glazes from non toxic materials. The reason for this is not that I am worried about going to court but rather that this is something people ask about when the buy pottery. That and it is safer/easier for me.

5. Is there a need for certification of the resistance to breakage of the manufactured goods if they are not machine-made?

I have never heard of such.

6. Can you recommend a ceramics supply store?

Cebex, EDAB keramik, Brännpunk (kilns, wheels, equipment mostly), Seemann (I have never bought from them but they are supposed to be good).

All in all, you are good to go, make your stuff, sell it and so on. If you plan to sell a lot and make a living off it you will at some point have to pay taxes. The easiest way to do so it to start an "enskild firma". Call Skatteverket (the tax office) and ask them what you need to do. They are usually very happy to answer questions from people wanting to pay tax so it should be easy.

Hej Bjorn!
Many thanks for the detailed answer. You have helped me a lot.
Best regards,
Stefan