The seemingly constant record-breaking sales is certainly one way to measure the success of the certified pre-owned market, but further evidence can also be found within the traction CPO is generating online.

Take Autotrader, for instance.

Used-vehicle listings on its website in May climbed 4 percent from the year-ago period, while certified pre-owned listings jumped 12 percent. Compare that to May 2014, when CPO listings increased 8 percent year-over-year.

So, at least with that particular metric, the upward slope has gotten a good bit steeper.

Autotrader also shared the 20 CPO models generating the most searches on its website for May, as listed below.

General Motors led the way with five of its models in the top 20 and Toyota was next with four. Ford had three models on the list, including the No. 1-ranked F-150.

1. Ford F-150

2. Chevrolet Silverado 1500

3. Honda Accord

4. BMW 3 Series

5. Mercedes-Benz E Class

6. Ford Mustang

7. Jeep Grand Cherokee

8. Porsche 911/911Turbo

9. Toyota Tacoma

10. GMC Sierra C/K1500

11. Jeep Wrangler

12. Chevrolet Corvette

13. Chevrolet Camaro

14. Chevrolet Tahoe

15. Toyota Tundra

16. Mercedes-Benz C Class

17. Toyota Camry

18. Toyota Highlander

19. Ford Explorer

20. Volkswagen Jetta