I fear its possible now! Bitcoin is already too diversified! Now it is nearly impossible for single seller or buyer to influence price of Bitcoin in any way! It moves greatly owing to forces of market i.e. demand and supply! It has nearly reached the state of "Perfect Competition".
You cant but dont forget that there might be someone who can. It is not really a big deal for someone that owned alot of it, afterall the current price is quite cheap for some rich guy so it is easy to actually see someone that has around 500-1000 btc and let that alone , imagine what a group of those bunch could do to the market
I still got a doubt person holding even 500-1000 btc can mobilize the price much! Think if he stock those btc instead of selling them, which in case of normal commodity can make the price rise but in case of btc will not effect price much. Since there is no limit in its supply right now! Moreover, if he go for mass sale, he for sure consider present rate, thus, contribute nothing but neutralising impact on price!
Bitcoin is not an essential commodity. Its an investment item. Holding or selling by single person will not effect the total market unless he is going to do that at insane amount. Example- selling 500 btc now @ $900 each
I think selling 500 BTC will be nowhere near enough
To correctly assess the effect of selling this number of bitcoins at market price you should take into account the total demand across all major exchanges that would equal this figure. If you just start selling these bitcoins at one exchange that will likely cause the price to go down significantly but ultimately you will be selling below the current market price and arbitrageurs won't miss the opportunity to profit off you. On the other hand, though, selling enough bitcoins may instigate panic selling and then you will be able to buy back at lower prices (and thus profit yourself)