If you want to get your items back, you have to buy them, which gives the money to yourself, but of course you still have to pay the tax. So you have 100 swords on the market for 10 gold each with 10% tax, you buy them all and end up losing 100 gold.
As far as I know, nothing happens and items can be up on the Town Center indefinitely (this needs to change to a better system, because searching through the Town Center is cluttered with nonsense and -2 stones for 99999 gold )
If they do sell, I believe the money is instantly transferred to you.