How much iron would a magnet pick up on the surface of Mars? Would it be rich in iron particles like going to a playground on earth? Would it be clean enough to take to a refinery and melt down, or would it be too rusty, aka oxidized? How would it have gotten so oxidized, given the currently low levels of oxygen?
Mars' atmosphere is very thin, but scientists think it was once much thicker, so the iron on Mars oxidized millions or even billions of years ago. Any new iron sources exposed by wind, meteor impacts or from meteors themselves will over time still oxidize as there is a small amount of oxygen in the atmosphere.
Rust is usable as a starting point. You can smelt it with carbon to convert the iron oxide into iron.
The common iron ores found on Earth are iron oxides.
Rust is less ferromagnetic than iron, but can be picked up with a magnet. You have to gather a lot though, the dust layer is only a few mm thick.
The iron oxides Fe3O4 and Fe2O3 are magnetic, only FeO is not magnetic. Read here https://en.wikipedia.org/wiki/Ferromagnetism about magnetic properties of iron and its oxides.