A good place to buy second hand folding bikes is Ebay, particularly if you are looking for the real quality end like Brompton folding bikes. I say this from experience as this is where I bought my bike and the impression I got was that people didn’t ever own a Brompton by accident. A Brompton owner has done his research and has decided that he wants a quality item and has spent quite a lot of money buying one. In my experience every Brompton seller had a pretty good reason for selling that made sense. I suspect that this is also the case with the Dahon folding bike owner too.
If you want to get a good idea of second hand folding bikes prices you can try using the Advanced Search in Ebay and looking for the ‘show completed listings’ option. This will list all the bikes in your query, the ones that sold are listed in green and the ones that didn’t sell are listed in red. This gives you a really good idea about what you can expect to pay for a particular bike.
Some of the cheaper bikes like the £100-£200 range from Argos or Halfords may not be worth buying second hand as they are cheap anyway. The reason I bought a Brompton was because I realised that the second hand value was extremely high and seemed to have nothing to do with the age of the bike, so although it was an expensive second hand bike, I wasn’t worried because I could probably sell it for the same price as I bought it for. I believe Ferrari owners will tell you the same thing too 🙂