I am a fan of saving money HOWEVER...
CAVEAT EMPTOR - "Let the buyer beware."
I say this not as a paranoid or one who would discredit manufacturing quality of Chinese origin - I do strongly encourage BUY USA when at all times possible though.
When something is on sale for a substantially reduced amount there is almost always a reason. In the case of core refurbished items there is a wide range of quality and life expectancy from different vendors. It is also possible that these cores were obtained from discards of other vendors.
As an example, I think a GREAT MANY oil coolers that have failed shortly after installation were purchased for a "DISCOUNT" on eBay. My proof is not backed scientifically but is based on the frequent number of postings that relate a common thread, "I got a great deal on eBay for an oil cooler and I'm going to do my EGR delete this weekend."
These oil coolers could very well have been the discards from an EGR cooler replacement TSB procedure (which calls for the oil cooler to be replaced regardless of the diagnosed capability of the oil cooler...which is usually the reason for the EGR cooler's failure). It is not hard to clean up the exterior of an oil cooler and - raise your hand if you've done this - HOW MANY FOLKS HAVE EVER PRESSURE CHECKED AN OIL COOLER PRIOR TO INSTALLATION?
(PGreenSVT and Bismic put your hands down now please!)
What I am saying besides the obvious, "If it looks to good to be true, you are right" is that there are REPUTABLE vendors - check here for experience with an eBay vendor first, read the feedback too. But you get absolute assured quality from reputable vendors like a trusted Ford dealership that services PowerStroke Diesels (not all of them do), from a trusted mechanic (who probably doesn't shop for parts on eBay but deals with the reputable dealers directly...and yes, those reputable dealers usually have eBay stores - remember I'm not slamming eBay or discount stores).
This is a long post - maybe it's more of a cautionary post in general than a glow plug warning - but I think it's worth saying that if QUALITY and RELIABILITY mean something to you then why buy GENERIC (a part of unknown quality and reliability) to save a few dollars when you're talking about ensuring the SAFE and NECESSARY (for livelihood...it's your work truck in many cases right?) operation of your 6.0L engine valued over $14,000?
You get what you pay for.
Be safe. Be smart.
Don't be penny-wise, dollar-foolish!