With pokerstars, it's a third party that buys your chips. This is because they somehow find a way to sell your chips to other people. As long as you settle the deal outside the pokerstars client (i.e., email), pokerstars doesn't care.
http://www.psplaymoney.com/sell_chips.htm will buy your chips for $12.50/million (you can sell them in 100k increments), I have actually used them before, and I would consider them trustworthy. (Of course, if that link shouldn't be there, feel free to get rid of it leaders)