Ok, I had the same problem, but I managed to fix it (I suppose)
Found a website with offline downloads for each version
If you download them and put in the android-sdk / platform folder named as "android-APINUMBER", for example, version 2.1 will be "android-7", then you will restart the manager and they will be shown as installed, hope that helps, as well as I hope they make them downloadable again.
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