Rather "You all are in the market"? Because it's what Duo accepts, for the other places of "all", it seems correct, but I doubt they accept it.
You should report it, to see.
Penny, you are correct. I have the same opinion. however back translating the English sentence to Latin ... you are at the market....it would then read in Latin : foro estis ( without the "in") so i guess one has to translate it from the Latin... you are "in" the market..