Being a newbie to Macs from a PC user for eternity I was disparing with the Garage Band in App purchase issue. Non of the suggestions worked for me. There was no accept option, the purchase was greyed out, re-entering my account info did nothing and making a first purchase on the new machine did not help.
However, I noticed that if I opened up the instruments the in App purchase was not greyed out so I proceeded from there. It looked as though it was going to work until the point of purchase when an error came up saying that the original was not purchased from my account.
A bit of lateral thinking made sense of this as the software was preinstalled! My solution (after a back up) was to delete GB and then go into the store to "purchase" the free version from the App Store. I was greeted with a message to the effect that a version had already been installed so did I wish to
"purchase" ? I replied yes and GB was downloaded against my account and I could then make the in App purchase of the full version. QED! GB is working with no problems now.