This may not be what you want to hear...But I will tell you how it works...(I own a used car dealership)
You buy a car on monday - getting a good price - you pay Retail price for it.
You decide on Thursday (same week) that you didn't get the car you really wanted. You talk to the dealer and guess what?? You can now trade that car in, but only for trade in value. See you have only had it 4 days, but its doesn't matter how long you have had the car, dealers wont pay "Retail" for a car, when they can go to the auction and pay rock bottom prices. This is why its never a good idea to trade in a car to the dealer. The dealer is the only one that will come out on top. So if you paid $18k for scion a year ago, my guess without knowing mileage and all the options on the car, which make a difference...I bet you get offered $12,500 for your Scion, at most.