Always a risk you are right about that. I guess if it's not Mark 2 or fast enough, I will get the unrecognized media error message. What's the best price anyone has seen on 16gb Mark 2's?
16GB Sony Memory Stick Pro Duo (Mark 2) are currently $64.95 at B&H.
The PCM-D50 was released before Mark 2 sticks were re-branded like they are, so Sony's little compatibility table looks like it was updated for recent times (it doesn't just take Mark 2 or HG sticks).
I've heard from numerous reports it runs fine on Memory Stick Pro Duo not labelled Mark 2 (I am using a Mark 2 card, however). The manual itself says "Memory Stick Pro HG-Duo" and "Memory Stick Pro Duo (High Speed)" which nearly all the regular ones are.
According to Wikipedia
http://en.wikipedia.org/wiki/Memory_Stick "All Memory Stick PROs larger than 1 GB support this High Speed mode".
So you should be fine with a regular Memory Stick Pro Duo of recent vintage, though it seems to make sense for Sony to push you into buying a card branded Mark 2 to simplify their table, even though these cards didn't even exist when the PCM-D50 was released.
http://www.memorystick.com/en/memorystick/feature/index.html#p02The Mark 2 sticks, incidentally, just specify a minimum write speed of 15mbps, which isn't anything special. Remember these are megaBITs per second, not megaBYTES.
eBay and memory sticks: just say no.
It's a surprise to me that Sony (or Sandisk or whoever) can't buy some fakes, prove that they are to eBay, and shut every seller down who deals in the stuff. They are hurting their reputations by letting it continue.