I think a good starting price for the Twizy would be between three and a half and four and a half thousand. I think four and a half to include the power box and assuming that the entire vehicle is in great condition, I think you may have someone bite at it.
Although I think the Twizy may actually hold its value indefinitely. I've seen some of these go for less too. Put a starting price and see perhaps what interests you get.
One bit of advice, make absolutely sure that you get the paperwork sorted for the battery before it is sold. Otherwise you could get caught out.