I've run several events like this over the years, and they usually have about 100 people show up for them. We run them a bit differently though.
-Players put items up for auction, there is no minimum bids, everything gets sold that day. Buyer pays the sales tax, seller gets the auction price in store credit.
-It's highly emphasized that this isn't the place to sell your nicely painted minitures, or things you love and want to keep. It's where you dump your junk, your old armies, your bitz, the stuff you painted when you were 10 and don't want on your shelves. People can turn in literally anything GW to be auctioned off, along with scenery, figure cases, paints, etc.
-Seller gets rid of stuff he doesn't want, gets new stuff. Store takes in x dollars in cash, gives out x dollars in merchandise, just like a regular sale. Buyer gets a deal, and paid what he wanted to pay for what he got. It's a 3 way triangle deal.
-Yes, the store makes money. The store also spends a week taking in auction items, paperwork is insane to keep it all straight, and I yell for 4 hours straight to auction it all off. Lot's of work on the stores end.
All in all they are fun events.