I sold the Medium House I bought for my FC recently after they quit to play Wildstar. The Rank 8 FC, Medium house, and all furnishings for 30mil (only cost 20mil.) It sold for so much because it was in Ward 1 and no other Mediums were available on the server. Anyways, trading that amount of gil becomes a problem as there's no safe way to do it. In my case, the buyer joined the FC, put 15mil into the FC Chest, and then paid the other 15mil after I gave him leadership of the FC and house.

The buyer could take your house and /blist you after giving you only half the price. The seller could refuse to hand over leadership after receiving your gil. Just be mindful of these possible worst case scenarios. Not many end-game players would risk destroying their reputations or getting their account banned over stolen gil so it would probably be best to check your buyer / sellers out on Lodestone before jumping into such a huge financial decision.

Some dark souls are out their waiting in the cesspools of the internets with sporks in their hot pockets and cheetos on their keyboards waiting to hit pre-made insult macros after they scam you.