Serfs were not slaves, they were basically tenants, most likely their family worked the land for generations and generations, paying taxes to the lord who owned the land. So if the land changed hands, they'd likely stay on the land. It's inaccurate to think of them as slaves, but if you consider them tenants, may give you a better understanding of their relationship with the land and lord. No they could not be sold. Population growth was indeed a problem. As families had multiple sons, they split their land for their sons to inherit. Slowly lands would become to small to support families, so sons would be pushed towards the priesthood or working as a trade/skilled worker, or joining the military. A lord wouldn't really be the one dealing with overpopulation as long as he was still getting paid his taxes, but he may choose to hire more servants/workers/soldiers in a time of overpopulation (supply and demand rules means that during these times these people cost less, so lord could afford to hire more)