Choose a reputable seller who knows Macs, tests and guarantees compatibility with your model Mac, and offers a lifetime warranty and a no cost (no restocking or shipping charge) return if it doesn't work.
NewEgg fails 3 or 4 of those 6 criteria. I would also avoid eBay sellers - how would you get support or warranty replacement when they vanish?
Be aware that generic Crucial and Kingston (ValueRAM) memory sold by 3rd parties (like NewEgg) is not guaranteed compatible with your Mac by the manufacturer - only their Mac-specific model numbers are compatibilty guaranteed.
MR Readers in the USA often recommend Crucial, OWC and Data Memory Systems.
DMB: Not true: Apple has never used Corsair brand RAM in Macs (or Crucial). Apple variously uses Samsung, Hynix, Micron, IBM and Elpida OEM RAM depending on the machine and their supply contracts. Corsair has does not guarantee compatibilty of their ValueSelect RAM in Macs.
Thanks
Trevor
CanadaRAM.com