FanSnap would argue Craigslist is fine for price-sensitive fans who don't need to go to a particular show and who are willing to meet and negotiate with other individuals, pay cash where necessary, and run the risk of buying a fake ticket. (Although the only time I've ever seen a fake concert ticket was in 1989 on the streets of Manhattan, when I bought a very realistic counterfeit to a Jane's Addiction show at the Ritz.)
Fans who want a slightly more convenient buying experience might go with eBay, where they can use PayPal and rely on seller ratings, while fans who absolutely need a guaranteed ticket can go with a marketplace like StubHub, which offers a toll-free customer service line, money-back guarantee, and other benefits. FanSnap operates on an affiliate-oriented revenue model, so it gets a commission from sites on which sales are made.
I ran a search on FanSnap for Pixies tickets, and it found more than 70 listings for the sold-out show this Friday, compared with about 30 listings on Craigslist. (The Craigslist screenshot in Tuesday's post showed only listings that had been added on that day.) Prices were similar to Craigslist--lower in a few cases--and the site has some great design touches, like a seating chart of the venue that maps tickets to particular locations. Craigslist, of course, is purposely and resolutely lo-fi. I still think there's something refreshing about dealing with a real fan, face to face, but I can see reasons why others wouldn't want to.
Correction at 8:00 a.m. PDT, Nov. 12: This post mischaracterized how FanSnap aggregates ticket listings. The site uses live feeds from its sources, which allows ticket listings to be updated immediately as prices change.