Can’t find beach items on SimCity Buildit

Those prices are set by the seller of the item. When you go to the exchange you are given the option to set the price that you're willing to accept for the item.

I've also found that if you set the price too high and no one buys it, eventually the system will buy your item and your selling spot is freed up for you to sell more items.

Here is what I received from EA:

Please note - the figure of 20% tax is calculated from levels and ratios for happiness and population, it's not as simple as happiness multiplied by population! A 20% tax rate is not simply 20% of your total population number in Simoleon form. If it were this simple an equation, early level Mayors would receive barely anything and find it a real struggle to make money for a long time, whereas higher level Mayors could buy everything in sight and would dominate the Markets. We didn't think this was very fair, particularly for a free-to-play game, and the balance of the game would have suffered too much.