I haven't checked the pricing on cars in a long while, but as a new player, I kept a small bit of cash in cars that could be sold immediately to City Docks. Essentially 100% liquid asset.
Back when it mattered to sell back to the city for net worth, I 'flipped' cars at a small % loss to Docks price. It requires lots of clicks, but it wasn't uncommon to make a few dollars profit.
cars is actually a very good idea to store play money. when they are at the right price they sell fast on my bazaar so they are a excellent way of storing money. a good trader will always buy them and some people collect them so if selling them on your bazaar or the docks is unappealing you can trade them on no problem.
Thanks for letting me know how and how much I'll get mugged and not having a protective free form of banking outside of investments and expensive vaults after a palace or better. Maybe you want us mugged lol