Can i sell my gold bars..?

It is specified very clearly in the Terms and Conditions. And of course it is fair. This is not a gambling app, it is a game.

Anything else you buy with real money doesn't get bought back by the shop where you bought it for no good reason. If you buy a Monopoly game in a shop, open it and find all that lovely paper money you can't go back to the shop and sell that either.