OEM is a bullshit term so ignore it whenever you see it used, by anyone, anytime.
iFixit does not compete on price so it can be assumed that they are offering something of quality.
eBay does compete on price to the lowest common denominator, so assume that "COMMODITY" items on eBay are $@$*, across ALL INDUSTRIES.
NEVER buy "commodity" repair parts on eBay. Random plastic piece that sits over a wifi card, sure. iPhone screen or Macbook battery NOOOOOOOOOOOOOOOO
I would buy a part from iFixit before buying it from eBay unless it was an eBay vendor I knew personally. Even then, good companies have their good pile(for their company website and wholesale users) and the $@$* pile(for eBay so they can compete on price). So be careful on eBay.