Just wanted to let you know. If the sales person at Carmax told you it would be 400 to paint the bumper, that doesn't mean you can take it there and have it painted for 400. I recommed getting estimates around the area from paint shops. Especially if it was a sales person who told you that and not a buyer. Simply because as sales people they use that with your appraisal to try to get you to sell it to them.
I only say that because I worked there, so just check around the area for some other places to paint it. Good luck with the sale though!