We found this on a list of Budapest flea markets. Open on Saturday mornings (possibly also Sundays ?) - took the Metro to the end of the line (Újpest-Központ) and it was about a hundred yards from the station.
The outdoor flea market had about a couple of hundred tables, with people selling EVERYTHING. Prices as expected were cheap. Seems to be off the beaten track - saw zero tourists. It was a nice day, and the square itself was very picturesque.
There are also a couple of (indoor) markets next to the square - mostly food (fruits, vegetables etc) and flowers, some bric-a-brac as well. Also some places to eat.
We liked it - a very nice look at a slice of local life, off the tourist path.