Taking it to a dealer gets you a much lower price
Please remember that while this is true there is a perfectly valid reason for it. These guys will have to flip it for a profit after all, and that's not easy for anyone in this day and age.
Good to do your homework though, there's nothing wrong at all in having an idea what something may be worth.
IMO most main dealers would only be interested if you traded it in for another car, (and you don't mention if you want to replace it).
The reason is that if they sell it off their pitch they will have to offer a warranty on what is, (despite low mileage), an 8 year old car, (no offence intended).
Even if you traded it in a main dealer would very likely just send it to auction.
So with this in mind they can only offer you an amount that gives them a chance to get their money back when the hammer falls.