Yes, but never forget that an index future is networked with the "real world" under it. If too many people sell, then arbitrage computers go in and buy, selling the stock that make the "real" index at the same time. So, the index future price can not derive too much from the "fair value". Fair value being the spot index plus or minus carry charges till contract expiration. For the index to move down, longer term, you need not only a lot of sells on the index, but also not enough buying power on the underlying stocks because the sell pressure on the index would spill over via arbitrage into the stock market.
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Per exchange regulations - yes, sells for example are at best price, so all puchase orders msut be exhausted before the next worse price somes. BUT: taht can go awfully fast especially when market amkers disappear because of carzya moves. I ahve seen imes the "next best" offer was 100 ticks away in the dow index (during the flash crash). Price jumping 150 ticks. Because "orderly market" is a very funny concept when basically volatile times come and the market integrity breaks down.
You must have a willing buyer to sell, you must have a willing seller to buy. When times seem great it can be tough to find sellers, when times seem horrible it can be tough to find buyers. When prices seem to high to those who already bought, expect a rush of sell orders to overwhelm demand. When prices seem too low to those who have not bought, expect a rush of buy orders to overwhelm supply. When the majority of the market is on the wrong side, expect the market to move further against them than you anticipated.
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