Unless you have time & money to burn, you really should never never never trade your car into the dealership. It's the worst price you're going to get along the used car supply chain. And even if you think you're getting a good price, it's because you're probably pairing it with buying a car (and paying too much for the car you're buying).
The worst-case used car dealer supply chain works like this (going backwards):
* You buy a used car for $20,000.
* The used car dealer bought it from a wholesaler for $19,000
* The wholesaler bought it from a wholesale auction for $18,000
* The dealership who sold it at auction paid $16,000 as a trade-in.
(*I saw worst case because not all of these steps happen each time)
Note the chain begins with trade-in -- because dealerships know that people who are trading in are pressed for time and want a no-hassle deal. But in doing that, they're giving up $thousands to each step along the chain.
So when selling a car - going direct (or using a service like InstaMotor) is always going to net you more $$ than trade-in.
And now ya know :)