Get a quick quote from webuyanycar.com - that will give you a rough idea of how much you could get for a car.
Selling a car privately can be a lot of hassle, part exchange is easier but you don't always get the best money.
A main dealer will not really want your 11-year old car as it's not the sort of thing they will want to sell on their forecourt so they will just send it straight to a car auction, hence the low price they will (probably) offer you for it.