Once the sellers have sold, they cannot sell the same coins any more
No, but the buyer could. And his buyer could. And so on and so on.
If the buyer sells at loss, he cannot continue for ever (he is out of business at some point). If he sells at profit, the coins get redistributed to new stronger hands.
1. If the buyer sells at a loss, it means he doesn't want to sell at a
bigger loss. See all failed alt coins; see DAO; See Beanie Babies.
If your reasoning is sound, it should work for those too.
The buyer doesn't need to "continue for ever (he is out of business at some point)." He has unloaded his failed investment, minimized his losses, now it's the next guy's problem.
2. If the buyer sells for the same amount that he bought it for, it's called "using Bitcoin as money, as intended by Satoshi."
3. If the buyer never sells, the money spent on bitcoin is simply lost money, as good as flushed down the toilet.