That's their problem if they choose to sell lower than the going rate. The two you reference--those sold quickly. Look at the sale just before and after those two--they sold for $950 and $960 respectively. Those are the prices people SHOULD be shooting for.
And the prices for the 24-105 have in fact gone up since March:
Source?
Going against what I said earlier about 24-105 providing a good range, I might be replacing my 17-50 with a wide prime! 
