how is this possible?

[Rufalus]

thats no investment, thats stupidness

I think so too. You can't make a solid investment when S-E are able to change the game at will. Spending a huge amount of gil on an item with the intention of reselling it many months down the line can easily come back to bite you in the ass when conditions in the game are altered to make that item common and worthless outside of your control.

[Habbit]

he is obviously a gil seller/gil buyer, transfering money over the marketboard instead of mail/trade to make it less suspicious, you can't really control the market on stuff like the nutkin(if it now was the nutkin), it's pointless and really really risky, because you wouldn't be able to sell all those for a very long time.