Things I should know before going into Carmax?

Someone you know is into cars. Fixing, modifying, buying and selling, etc. Have him/her go with you. At the very least take a long test drive on city streets, freeways, smooth roads, bumpy roads to see how you like the handling, braking, ride, sight lines, mirrors, seats, etc. Try 3-4 different makes that interest you so you can compare them on the same roads.

I like to start by examining the interior of the car specifically the condition for heavy wear. Many a car was a rental car and the rear seat is almost new were the front drivers and partially the passenger are warn in well. I check under the hood for odd coloring on the block and silicone on bolted components to the block. any car with mileage will have needed some service and a shop can stream clean an engine to look nice but be pretty bad. Some shops spray a detailing spray on belts and hoses hoping you see a shiny engine rather than realize they sprayed them to hide the wear and rear of the hoses and belts. Low mileage engines will look pretty close to original but likely washed and often I can see dirt struck behind less accessible spots by the detailer. This does not worry me as they simply don't want you to see the dirt that was there. I however look closely for darker more imbedded dirt as this would tell me it was not looked at much by owner. A good owner checks there oil and coolant levels regularly. They would spot and either fix or wipe off and watch it to determine where it came from.

Have someone with you who has bought a car before with you.

Edit: There is a problem here. You are desperate and sales people will sense that. You can pretend your friend as the buyer and ask questions. You, not under pressure can see more clearly. Unfortunately, questions to ask like, why are you selling the car and do you have maintenance records will not fly with sales people. If you see something you like then you need to know a fair price. Compare it to others who are selling the same model on like Craigslist.

Always remember that you can walk away, but then you are desperate...

There's really not much to buying a car at CarMax. They don't haggle. The price on window sticker is the price you pay, there is no negotiating. That goes for any trade in you may have as well. They'll offer you price on the trade in and that's what you'll get for it, again no negotiating.

Generally speaking, you will pay for any given car at CarMax vs. the best deal you could potentially negotiate at a normal dealer.

they are going to try hard to sell you a car.