The best way to offer these for sale would be through auction. I recommend contacting Leslie Hindman Auctioneers and Susanin's in Chicago. It is free to contact them through their website. Here are the web addresses for you:
It is a contemporary artist market, so essentially the price will be based on a percentage of the high retail. In general, works by contemporary artists do not exponentially rise over time after being initially purchased, that is an anomaly for a very, very small percentage of current active artists.
Here some sale examples, one valued at $600 sold for $255:
And here is another that sold for $145:
A few others old for under $100.