Can you trade in used car for used car?

Can you trade in used car for used car?
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; andto ask to have the car inspected by an independent mechanic before they buy.