Most of the time for large scale traders who deal in huge volume, and they know buying directly from the market would bring about some huge growth which the same will happen when it comes to selling the market, OTC way is always the best way to load up or offload as the case may be.

However, it does not necessarily mean, because no matter the volume you are buying, OTC is available to practically everyone whether you are a small scale trader or large scale trader, but in most cases, for large scale traders, OTC is always the best and ideal way.