I've bought two vehicles at Carmax and used my financing both times and I agree they are very competitive. I never found any used cars at any dealer that were close to what I ended up paying. I got a loaded Sienna and an RX 350 F-sport and it was really hard to find anything in private sales. Both cars worked out well and because I had done my research, I got $2k off the Sienna because they listed it as a Limited and it wasn't. It had all of the upgrades except one thing (the reverse sensors, had the camera). I mentioned that it couldn't be a Limited and they said I was right. I loved the break on the price because I knew that was literally the only option that was different. Also, I did a quick search of the Carmax's near me and they had 28 cars under $10k. They don't have a huge assortment, looked like just under $8k was the cheapest.
Anyway, enough of the story, but I've had really good experiences at Carmax and one thing that worked really well was all the pictures. I looked at every RX 350 that had what I wanted and looked at all the leather, dash, etc. just to see scuff marks and anything really minor. All those cars were identically priced, but I found the most perfect one I could and got that one. It was nice being able to check out all the different SUVs to narrow it down to the one that drove and felt the best and then find the exact one you wanted and have it shipped. I was really pleased with the results. Heck, my Sienna was competitively priced such that I bought it for $25k with high 20k miles, drove it for 100k miles in 5+ years and sold it myself for $11k. Not a bad cost of ownership for a car we really put through the works with 3 kids.