The only sure way I know of to get single carts to print single swaths is through a nozzle check. "Inputting" RGB color levels has to pass through the driver, which then can mix inks in a proprietary way.
If you cant test it, either take a risk and buy it (negotiating the price), or don't. It's like buying a used car without being permitted to take a test drive.
Did you recently change ANY carts?
I'd also suspect the ink, not just the printhead.
It would be great to force a Service Test Print from a Canon Pro-100 (where every nozzle can be individually examined through the "grids". Anyone ever been able to do that?
Since we have had a recent report of a purge system issue with an Epson printer, you might try flooding the purge pads (the station where the printhead parks) with Original Windex to see if you can establish a standing "puddle" on the pads, put the printer through a cleaning cycle, and see if...
First time I've heard of an Epson Purge Unit failure...sounds quite similar to some Canon failures (typically loose or clogged tubes).
As I said, it WAS air (lack of ink in the nozzle chamber). I did not expect a purge system issue with an Epson. Now I know...
A nozzle check print on a PRO-100 (and most Canon consumer printers), prints light blocks of pastel colors, that "should" look completely featureless if good. No streaking or banding should be visible at all.
A service test print (easily to print on older Canons) would show EACH NOZZLE'S...