If I print in duplex mode, I get a dirty mark across the page on the
second side. It's the same whether I print from the tray or from the
manual feed.

Could this be a cartridge problem? It's a years-old remanufactured
cartridge, but the dirty marks have only just appeared.

Reliable cartridges now seem to be somewhat scarce and expensive. Would
it be more economical in the long run just to buy a new printer with
cheaper cartridges? Brother? Samsung?

Perce