No you do not have to pay cash. These auctions are different then trustee sales. Yes the buyer does have to pay 5% on top of the winning bid. So 327K is the winning bid so tack on another 16k give or take.
Remember the auction is held by representatives who basically represent the lenders. They do all they can to get the winners to finance with them. I do not believe it is a requirement for the buyers to finance with the lenders at the auction. I do not know how long the contingency periods are for any auction winners. However, I believe the contingency periods are ZERO!!! Thus if you win, you pay or you lose your deposit if you have remorse a few days later.
I know of someone who bid on a home at the June auction, and they had the winning bid. When they really inspected the home post auction they found there was water damage and to make a long story short wanted to pull out. They had a prolonged exchange for several weeks and after some very crafty negotiating by them, including some serious threats of legal action, they did indeed get the full deposit back. They really worked at it and I gave them some help.