If you google up a translation of the word Loppis it says flea market, and its a general term that can cover car boot and flea markets too. I think the Swedish perhaps have one word for their old junk, whereas in the UK we like to narrow it down and categorize the types of junk we are selling ha ha !
The Windmill owners also have a studio for ceramic making and run courses there. The other Loppis has a facebook page, they are called Mormors Lupbod.
Grillby can be found just off the E18 out of Stockholm, just over an hours drive. Its just before Enkoping.