I seem to recall another rumour regarding Sale of the Century is that everything of the US run exists, just that it's in storage in the Grundy archive (in Australia).
I wouldn't read too much into the fact that GSN is only running the 88-89 episodes right now. As with their recent purchase of the first 50 shows of Press Your Luck, I suspect it's only an "archive dip" to test whether the show rates high enough to warrant leasing any additional episodes. Those first 50 PYL episodes ran over and over about 4-5 times (I think) before GSN decided that, "yes, we'll lease some more."
Maybe they'll do the same with Sale, (of course, if the "next" bath of sale is the syndicated run, then the whole "everything else is wiped" theories will resurface).
Another possible reason as to why GSN chose those specific episodes: A). to be able to show the end of the series or B). maybe those episdoes (and the others that were run on USA) were converted to a more modern (digital) tape format, and all the older episodes (the ones not aired on USA), are still on ancient formats that would have to go through digital conversion/restoration before they could be aired.