The tickets on the web site sold out for week 2 around 6 months ago so I bought tickets for week 1 and put myself on the waiting list for week 2 (I know people that always go to week 2 so I really wanted week 2.) Then last month they called and a few people could not come to week 2 and were trying to sell their tickets so I bought tickets for week2. So now I have 6 tickets and may sell a couple nights (and only go to 3 or 4 concerts)...but I just have tickets for myself and most people are with someone and want 2 tickets.
Last year there was a lady taking names of people that needed tickets (an ad hoc thing) and people were getting them that way by word of mouth. If I run into someone with tickets for sale I'll let you know. Sometimes you can even buy them at the gate...but I'm not sure you can depend on that. (I was sick one night and a friend took my ticket and sold it at the gate two years ago.) It is $150 for 3 concerts and people just sell individual tickets for the $50 they paid for them...they're not trying to make money on the deal. (Of course you also get to go to the bar-b-que and get a free t-shirt when you buy for the whole week.)