Stupid question time! I was wondering if houses that users sell are any different than the ones you can just buy outright (possible price differences aside)? Like, could they come with deco/plants/stuff already inside them? If not, is this a feature that might be added in the future?

They're identical. Once you move houses you don't really need them, but you get more PC from selling them to players than you do selling them back to the server. So you sell it for less than the brand new ones to incentivize other players to buy them from you.

But for the buyer, there is no difference.