Well posting year/miles/model etc might help.
FWIW - I'm selling mine in a few months, low miles a few mods but if I return it to a standard car (2005/37kmiles/azure), I'm getting offered 6700. That is a part exchange without even seeing my car. But even part ex is full of worms - as a high part ex price will most likely mean your paying extra or a premium for your new car. Point to remember is dealers need to make a living also - which I dont mind (unlike others).
With mods and so on I asked forum members what mine was worth - and prices were all over the shop - much the same as it is retail for 350zs. Reckon a lot to do with that is condition - a similiar mile/age car that is a standard low spec (cloth/non GT), nasty colour as one thats mint (decent spec) with a few mods and decent colour might vary with well over 1.5k.
So best bet - describe your car honestly, what mods it has and someone will pop along and let you know.
Dont rely on autotrader and such - cars might be advertised as such - but what they sell for is something else (again - back to condition/spec/miles).
If you have mods - strip them off and sell - you will defo make more.
For instance - milltek cat back second hand - 500 quid? Side skirt - 80 quid? D2 controller - 60 quid? Wheels - 400 quid? HFCs - 150 quid? etc etc . Already your looking at 1200 quid. Would you get that extra with leaving them on the car???? I wish!
Me?? I dont have the heart so mine will be sold with wheel, skirts, milltek and all other bits.