The Tomorrow Series Mystery

A while ago I talked about the fantastic Tomorrow Series when I had finished the second book. After that I went on to order the third (the first two were only like £4 on Amazon). I found that the third, fourth and sixth books were around £70-£120 on Amazon! I was in shock, they were only paper back books, and as I said, the first two and (after I searched) the fifth and seventh, were only about £4. Ridiculous!
    I searched round, and found that out of the whole series, those three books were almost impossible to get my hands on without having to pay a hell of a lot of money! I found an Australian seller on Ebay selling the Australian versions for about £10 each including postage and packaging. It was perfect but I very (VERY) stupidly left off getting them until it was too late and there were no more. But I needed these books so bad! And as my birthday was in a month, I wanted to ask for them as a birthday present.
     In the end, I found this site that finds and sells out-of-print books which had quite a few of the books I wanted. Unfortunately, they were still pretty pricey, but just about affordable for my birthday. I decided on getting the third and fourth books and holding off on getting the sixth for the moment. The two books came to about £30. One is an second-hand hardback library book from Austrailia and the other is from the US with a cover that I HATE. But still, it's not about the covers, it's about the books and I now can't wait to read them.
      But I still can't get my head round the fact that the third, fourth and fifth books can't be found in the UK when the rest can. It will have to remain a mystery for now.