In order for people to sell their btc is because it becomes unsaleable or falls so much in value. Than this investor could easily purchase all the btc`s in market but even if he does it, if people don`t show interest in btc the demand would be very low by leaving this man with a lot of btc but no volume.
Only a fraction of all the bitcoins that have been issued today are sold in the exchange market. The Bitcoin market is competitive, meaning bitcoin prices will rise or fall depending on demand and supply. Moreover, the new bitcoin will continue to be published in the coming decades. Therefore even the most ambitious buyers will not be able to buy all the bitcoins that are present today. However, this situation does not indicate that the market is not vulnerable to price manipulation; still does not require a lot of money to move the market price up or down, so so far Bitcoin is still classified as a volatile asset.
The distribution of bitcoins is very uneven. More than 20% of all bitcoins are on 100 addresses. Here are the detailed statistics, if interested