I had a coworker at my previous job do basically the same thing. When I asked her what she thought of carmax, she had nothing but good things to say about them and her experience.
That said, if you’re going to finance another car, get multiple quotes/lending preapprovals and see what rates and duration works best for you. The last 2 cars I purchased from the Honda dealership were financed through Honda with 0% interest (maybe I just got lucky twice in a row with promotions?). This was on a brand new Honda FIT and then again with my current used 2016 civic, so the dealership might also have solid lending options as well.