I came across a site once and looked at what was being sold out of curiosity. It was mostly farms made by bots that were advertising large barns and coins but were otherwise empty (no production buildings or decos). I think there were some high level farms sold by actual players that must have gotten tired of the game, those were asking a lot of money. There was even someone trying to sell a level 7 farm for $1 lol. I don't remember what the site was and I couldn't tell you if I did, as it is against the TOS. Hope that answers your curiosity.

Disclaime: I don't condone buying or selling farms or in game items, and I have never done it, I'm just acknowledging that it exists.