Which of the following is a requirement for dealers under the Highway Traffic Act?

Keeping a record of vehicle sales is a key requirement for dealers under the Highway Traffic Act. This regulation ensures that there is a clear, traceable history of transactions, which is crucial for both consumer protection and law enforcement purposes. By maintaining accurate sales records, dealers can demonstrate compliance with various regulations and provide an account of their business activities. This practice also protects consumers by allowing for traceability of vehicle ownership and history, which can be important in case of disputes or recalls.

In contrast, the other options do not reflect requirements set forth by the Highway Traffic Act. Warranties, maintenance offers, and selling only certified used vehicles may relate to best practices or business policies, but they are not mandated by the Act itself. Therefore, proper record-keeping stands out as a fundamental legal obligation for vehicle dealers.