I bought the 5350 because it was 50 more than the 3500. I'm nowhere near WiMAX and probably won't ever use it, but come on! It's 50 bigger!
Seriously, it was actually on a few bucks more. If it was much more expensive than the 5300, I would have gotten the latter. But anyway, the WiFi features of the two cards are identical.
As for why EBay is so much cheaper, I don't know for sure. All the EBay sellers seem to come from China and Hong Kong, and they all have thousands of EBay points, so they must work on high-volume, low-margin. The shipping is a longer wait as well, obviously.