In order to figure out whether you should take credit-cards for payment at your auction, my advice is to start by looking at your procurement effort, and what you think the range of guest payment totals will be at the end of the evening - and what you WANT those to be.
Easy example #1: you are running a silent auction only, with 130 guests and have procured 50 items from local businesses, worth roughly $3,000. 130 guests usually equals somewhere around 70 bid numbers, since most couples will share a bid number. You can anticipate that the average winning bid will be about 80% of the Fair Market Value, so that's $2,400.00 you are going to raise. The average guest payment will be about $35; the high-end big-spender probably spends $350. In that case, I wouldn't bother with cards - bring a cash box; tell your guests to bring their checkbooks, skip the fees.
Easy Example #2: you are running a gala with both a live and silent auction, plus signup parties and a $100/ticket, pick-your-prize raffle where you expect to sell 30 tickets. You expect 280 guests (150 bid numbers); counting signup-party slots, you have 250 items, at a total FMV of $40,000. Your average guest total will likely be $200 or so, but in this scenario, it is likely (from my experience) that very few guests will have an "average" total; 40-50 bid numbers won't spend anything beyond their ticket, 75 bid numbers will spend between $200-$400 each, 20 bid numbers will spend $400-$500; and 5 will spend over $1,000 each. In this scenario, if you don't accept credit-cards, you can reduce each of those total amounts by half or more. It's a no-brainer to take cards then.
I'm going to save here, and address another point in my next post.
Roger Devine