Sometimes I sell off games if they are unlikely to be played any further, but it depends on what the value is.
Right now not many Wii games have a high value. This is good, because I've been able to build a large collection at a very reasonable price. However, it also means I'm not keen on parting with the games in the near term because the hassle of selling (and possibly shipping) outweighs the value I feel they would receive by selling.
Xenoblade Chronicles (Wii)
£15.00
That's a steal of a price compared to going rates on certain large auction sites.