Can you trade in used car for used car?

Answer 1

yes.

In US there are Federal Trade Commission's (FTC's) Used Car Rule to comply.

The Buyers GuideA disclosure document that gives consumers important purchasing and warranty information, the Buyers Guide tells consumers:

• whether the vehicle is being sold "as is" or with a warranty;
• what percentage of the repair costs a dealer will pay under warranty;
• that oral promises are difficult to enforce;
• to get all promises in writing;
• to keep the Buyers Guide for reference after the sale;
• the major mechanical and electrical systems on the car, as well as some of the major problems that consumers should look out for; and
• to ask to have the car inspected by an independent mechanic before they buy.