It's really easy. Place a sign-up sheet at each item (usually in $5 or $10 increments). You can set the start amount at $5 or $50 depending on how big the item is.
$5______________________ $15_________________
$10_____________________ $20_________________
etc.
The person signs their name until a certain time in the evening when the winners are announced.
Winners have to check out and pay by cash or check (credit card if you have one).