I have seen and even purchased a used Geo 3 recently. Basic colors $1000 to $1100. The boost could add $50 and the anno maybe another $100. Total $1150 to $1250. Realize that to fetch top dollar you will have to wait for the perfect buyer. Additionally, the top end I quoted will be hard to get because it so close to brand new.
Piece of advice lots of pictures, quality pictures (if you suck at taking them or have a bad camera) then expect a lot lower. For a marker this high end don't even think about saying pictures on request or texting them from your phone! A link to a decent shooting video is a plus, too! If you are open to some trades plus cash more likely to get the stated value.