Buying large amounts of HBD is a chalange ... with the recent launch of the pHBD-HBD pool on Polycub, there is like around 350k in liquidity in the pool there.
The best place to buy large amounts of HBD probably remains the internal DEX https://wallet.hive.blog/market , but it needs to happen with multiple orders in a period of days, not one big sum.
What happens when a large number of HBDs are sold? Does it burn, does it become hive?
Selling and converting HBD are two different things. You can sell HBD on the internal DEX for HIVE or on any other exchanges where HBD is listed. If a big amount is sold at once HBD will dip, but most likely recover fast, because the blockchain offers a 1$ worth HBD through conversions.
When you make conversions from HBD to HIVE, the HBD is destroyed in the process (burned), and the blockchain gives HIVE, that is worth for 1$ price for HBD. With conversions one can convert any amount of HBD to HIVE without impacting the HBD markets.