macbook RAM sold in the apple store is about a squillion times more expensive than what’s available on ebay (brand new, but shipped from exotic places). are there any reasons why i should go for the apple chips?

No, you can and should buy high-quality ram elsewhere. Some people keep the original ram in case they have to send in the Macbook for repair, so Apple cannot claim faulty ram as the cause of their troubles. I sold my old ram on Craigslist.