I've got that in a main shop when I was buying something. I think you just get that message in you were just a second too slow when buying and someone buys it before you.

I think that it has to say Sold Out because when it says the This item was previously purchased and I go back to the shop the item is often still there. And sometimes I can purchase the second try.

I've been getting this too. I'm wondering if it has anything to do with the -1 I keep seeing in the token shop. It may be a new measure to make sure an item doesn't get sold twice? When I get the 'sold out' message and go back to the shop, there are none there, but when I get the "someone bought it" message, the item might still be there.