You can shop without a shared language
One nice thing about flea markets is that you do not need long conversations. If both sides know which item and which price, the deal works. Pointing at an item and saying “This one”, and asking “How much?”, covers most of it. “Ikura desuka?” (How much?) is also easy to understand.
A phone calculator is very handy for money. Type the number and show the screen, and there is no misunderstanding. Sellers sometimes type a number back on their own phone.
Bargain politely
Bargaining is part of the culture in Japan too, but demanding half price in a sharp tone will sour the mood. When buying two or more, asking politely, “Could you make it a little cheaper?”, is often a smooth way to agree. Pushing too hard makes the seller uncomfortable, so keep it friendly and within reason.
One word before you take a photo
Each item has its own history, and sellers can feel either happy or uneasy seeing their stall in a photo. A habit of asking first makes the day pleasant for everyone. Point to your phone and ask “Picture OK?”. In many cases you will get a gesture of yes. If the answer is no, do not take the photo.
Ceramics, lacquerware, antiques and old kimono are handled with particular care. Do not pick them up without a look and a nod first. Some venues have rules on photography, and permission may be required. A personal keepsake photo and a photo of goods may be treated differently.