Ebay is a very popular model kit exchange if you have the time to participate. You can search for recent sales of kits that match the ones you're selling in order to get a fair market value before you list.
If you prefer to sell the collection, there are people who will buy them. Look in Ebay and google topics like "model kit buyer", "kit collector", etc. You should talk to a few of them to get the best price.
Hope this helps.
Mike