Used dulcimers. That's a tough one.

The market for used lap dulcimers is mostly nonexistant. Unless you are part of a network of lap dulcimer players it is quite difficult to find one for purchase and even more difficult to sell one. Been there.

Of course there's always ebay. I myself have never sold anything on or purhased anything from ebay. I occasionally check ebay for dulcimers because people sometimes ask me the same question you asked. ebay normally has dulcimers I wouldn't take as a gift, but once in a while I will see a dulcimer by a luther I respect. I think that a number of knowledgable dulcimer players lurk ebay. The few decent lap dulcimers I have seen attracted bids that suggest they were made by people who know what they were bidding on. At ebay go to Music > Musical Instruments > String and search on "dulcimer".

A better bet might be to look for local groups of dulcimer players. Attend a meeting. Ask questions. See http://www.zsigray.com/md.c.index.html.

Also check the classifieds in Dulcimer Players News (http://www.whistlepig.com/dpnews/)

Where/how did you get the dulcimer you are currently playing? Repeat that process.

Be patient.