Generally I find that if you are a high level character and you are wearing expensive items, it seems to happen less. Having sparkles or something like sewer king seems to convince people that you are reasonably wealthy. In cases they ask though, show them the money. Then leave the trade window after they get a good look, unless they show the item. You made have to trade again (often there is some haggling).

People generally don't want to remove stuff from the cs without reason to help that you can afford it. On the downside, sparkling sets tend to make you a magnet for beggars, especially in the cs and forest haven.