I guess it's the economic environment that's encouraging an increasing number of people to consider selling the odd painting. This is an interesting exercise not only for the vendor but for the gallery they ask to help them. The commission is not a great incentive for a gallerist - usually a lot less than for the primary sale - but checking out how the market is holding up in the long term certainly is. Recent experiences suggest that it is possible to double your money in, say 8/10 years - better odds than most investments. The key is the artists themselves - if the work is of good quality, and the artist well established, then the chances are you will make money selling. This is good for the primary market too - because it encourages buyers, and gives gallerists some examples of what might be possible.
When all is said and done however, surely the best investment is in something you are going to love for a long time. The painting the kids want you to leave them when you go!!!