Where can I buy a used kiteboard, kite, harness and bar setup?

Try going to Cabarete and start with the kite schools

Check the AdScene paper, there is always Kite stuff available in it.

The market for secondhand gear in Cab is not great, prices are high and, if buying from schools, you know that their gear is very likely to have been well used.
That said, there are some good deals to be had, though often from people leaving and trying to get shot of gear that they dont have space in their bags for.
Have a good look round all the kite schools. There were a couple of boards for sale on the notice board at Kite Club a while back, though not sure if they are still there.
You do kind of need to know what you are after though...good luck!