IGB is practically worthless though. This happens even in forums/communities with no TBT system. Prices are horribly inflated, which is a sign of IGB having lost its value. The reason being that you got duping and powersaves, which means that you can make hundreds of millions of bells in under a day. And even without cheating or TTing you can still easily make hundreds of millions of bells every month (stalk market is risk free if you stalk that subforum for towns w/ high prices)
So after someone has been playing the game for a long time they really have no use for IGB anymore. Try to buy from them they'll say no to IGB.
This turns into a Domino effect. If people are turning down your offers because you only got IGB, the next time you sell something you'll try to sell it for TBT seeing as you had no luck with IGB. And then the people wanting to buy from you will also start selling for TBT because they want to buy stuff from people like you. And so on.