Not sold out as of this morning. Are you sure you're clicking on the right page? You have to go to the Pepsi Grandstand Concerts link on the upper right hand side of the main page.

The main SC State Fair page has links to 'Buy Tickets' but that's SC State Fair Tix (which are being sold currently at $5/person instead of $10/person at the gate during the fair...so get one of those, for sure!)

But the link you want for concert tickets is this one:
http://2010.scstatefair.org/entertainment/grandstand-concerts.html

ETA
Actually, I just went to the 'Tour' page on this (Avett) website, and I see why you are confused and where you are probably having the issue. The link they are using for "Buy" tickets doesn't even get you to the main SC State Fair website (so nothing I've said above probably even makes sense!).

So...let's make this really easy. Here's the link to the actual Avett show:
https://www.etix.com/ticket/servlet/onlineSale;jsessionid=1770455D0AFB58...

I was able to click and set up an order just now when I tried. Hope that works for you, too.