BTER.com although not the biggest, BTER appears to have the most stable and consistent platform. While there are reports of deposit/withdrawal issues, this is the case for any exchange. Here at Quarkcoins.com, we recommend BTER for your buying and selling needs, and is our go to exchange for our own personal trades.
Note: When creating your account, you will need to create 2 passwords. Remember both of these, as the second one is needed when you want to withdraw your coins.
Bleutrade is a large exchange that trades Quark. Its big advantage is that it currently offers many alt-coins to trade. So it is helpful to have an active account here when you want to trade obscure alt-coins. Bleutrade is our second choice for trading QRK. Granted they were resolved.
BTC38 is a Chinese exchange that trades bitcoins and alt-coins with a direct CNY/BTC fiat exchange.