How much is a GL 550 2008 worth at 120,000 miles?

If you go to KBB.com Kelly Blue Book, just enter basic info about your car, it's year, mileage, options, etc and you will get the current valye to sell the car yourself, or to trade in. Many times with the trade in, you will see immediate trade in offers, or offers to buy reflected in the quote. Hope this helps.

Sell it yourself. My experience with Mercedes Benz is they like to low ball your trade in at about 50 %. They have a lot of leases.