How do US customs brokers handle import documents?

Depends on the company greatly. Still, it is always better to use https://clearitusa.com/customs-consulting/  custom broker support. I really like that it is possible to do especially if you are buying a car from the auction. It is fast and the custom broker can answer all your questions.

In china, suggest you to hire a customs broker to handle import documents. For example, electronic product import clearance in China, it needs to provide 3C certificate. This certificate is apply by supplier. If no 3C certificate, product will be stuck in customs which can not import into china. Customs broker can advise where to apply document and how to prepare document in advance.

There are many opportunities for buying a car in the United States, but there are many options for buying an apartment, just look at USA https://realting.com/property-for-sale/turkey real estate - they are really worth it. They have many advantages.