What's going on with the price on dupes?

[aviator]

No need to sell something that someone else can just make in an hour or two (and can probably do a better job).

Maybe people can't be bothered to spend that time making these constructions and so purchase ones they like from other people. Likewise, it may be because they can't build to that level that they choose to buy it. This is why such market exists.

As Trevor Jackson said, it's an easy way to get your hands on a good build.