I would assume that if some item is returned with serious issue, OCUK wouldn't put such item for sale, as it still can be returned, but for a shorter period of time.
For things like CPU and GPU many people would return it simply because couldn't get a good clocking chip, or in GPU case, fan noise. Motherboards, simply because didn't like it, but there's a chance of parts missing.
More sensitive things as SSDs, HDDs and monitors, personally I wouldn't try my luck. Wrongly handled storage units may take a while to show the damage, if any happened. Monitors, you may not be picky, or your man cave don't highlight the issues, but more often than not they are returned for some "problem". Excessive light bleeding, issues with the screen uniformity, dead pixels. But as I've said, a monitor which shows excessive light bleeding at my office, using my specific calibration, may not show or bother you that much at your place.
Memory, I assume is due to incompatibility. Or at some extent people unable to overclock it, may try for a different model. Mine I had to spend few hours adjusting the CPU cooler to allow it to fit, but avoiding touching the side window.
Being as honest as possible, no one can tell you for sure why the item was returned, not even OCUK, as the customer may have feed them with wrong reason to return the item.
A clearly faulty item wouldn't be put for sale, if OCUK is aware of that.
If is just an open box item, or minor signs of use, fair enough, but the reduced price takes that into consideration.
And there's end of life items, great for those who want to give a bit of live to their systems, delaying a bigger upgrade.