I would strongly advise against buying SD cards or similar on ebay regardless of pricing. It seems like the majority of the fakes people get come from various sellers off of ebay, especially that memory that almost seems to be priced too well. Even that one on Amazon only has three reviews, of those, only two held onto the card, and of those two, neither actually made it a point to check and see if the card was legitimately 128GB or not. I would spend a bit more myself on a well reviewed card, or at least one from a recognizable brand.
This. A million times. I got a good deal at walmart for a SanDisk 32GB UHS-1 class 10 and it only cost me $10 because it was black friday. The only way you're getting 128 GB under $40 is from something like that. If you're willing to take a risk with 3rd party sellers and for just over $40, there's some generic OEM cards you can get at newegg: http://goo.gl/rldGR4
Just purchased a 128GB Sandisk card for 25. I'll try it out and if it's not what was described, I'm sending the sucker back. eBay has a fairly good guarantee but try to find a card with a recognizable brand name. Both amazon and eBay are filled with fake high capacity cards.