You can sell licensed Lego sets here all day long. New and used, complete and incomplete. I'm not sure about MOC's and custom builds. I haven't seen that option. Your best bet in my opinion is Ebay for those.
Do you mean assembled as in a genuine LEGO set that has been built and won't be dismantled for shipping? I suppose you could but would probably need 'request a quote' as a shipping option as the official dimensions are for the box. Also, as a customer I would be not keen to purchase a set like this for two reasons; 1) the fun aspect! I want the experience of building something, as it's not just about the end result, and 2) the completist in me would want all the correct parts, even those that are hidden within the structure.
I got a set from ebay one time, excited at the prospect of the build process, only to open the parcel and discover it was pretty much completely constructed (and the seller made no mention of this). Ended up waiting in another room while my partner patiently dismantled it for me!
Completely agree with Jay. Quite apart from quadrupling the postage costs, is there really a market for people who want to buy a lego set, but don't want to build it themselves?
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I got a set from ebay one time, excited at the prospect of the build process, only to open the parcel and discover it was pretty much completely constructed (and the seller made no mention of this). Ended up waiting in another room while my partner patiently dismantled it for me!