It's pretty much impossible for normal users, there are bids in the hundreds of thousands of gems... I can't even understand how someone can have that many.
The market was(is?) flooded with items everyone had but nobody wanted.
I'm not entirely sure, but I assume people just buy all the emoticons and backgrounds in the lowest price range (I think $0.01 - $0.03 is as low as it goes? But don't quote me on that) and then just turn them into gems.
If you buy 100 emoticons for $3, and each emoticon is worth 100 gems, you essentially traded $3 for 10k gems.