If they stop paying the mortgage, the bank can foreclose on their non-built house. Frankly speaking, the government is probably going to bail all of these people out; if they don’t the whole real estate sector could fall over. The practice of selling houses yet built is not limited to Evergrande.
In most countries, you aren’t able to even get a mortgage on a house the bank can’t put a lien on. The banks are actually complicit here, but they are mostly state owned, so the government will manage fall out here in the direction away from social unrest.
