Yes, Rock am Ring and Rock im Park were sold out the past years. Currently there are still enough tickets available for Rock im Park (170€ for 4 days!). Rock am Ring is sold out already.
I'm not too sure if there will be tickets on sale on the weekend. But if so, they will be very expensive! (for example: 250€ instead of 150€). Maybe postage costs will be lower than buying a ticket directly at Rock im Park.
edit: please check www.parkrocker.net for further information on Rock im Park They also do have an english speaking forum: www.parkrocker.net/forumdisplay.php?f=713