Well, whats the best way to sell any consumer electronic device?
there is no "Best Way", But, since you seem to be a pretty tech-capable user, try out gumtree. I Have sold a bunch of stuff there. no problems.
in response to your Price guides:
17" - The fact that its a 2009 model outweighs the fact that the 17" is now a "Rare" item. why do you think apple doesn't make it anymore? Why? because no one wanted it. the only people it served well were Media Professionals and the "Desktop Replacement" Crowd (I should know, I work with media on a macbook pro 15" 2010 model.) and even if you found a media pro who wanted one, im pretty sure he/she would be looking for a new one.
2.8Ghz - Now this is a selling point. although some may take issue at the fact that its not an i5/i7 processor, or quadcore, which everyone has moved on to.
8GB - Again, a good selling point, but what brand memory did you use? Macs are exceptionally finicky about the memory used, but if you used a brand like Kingston or Crucial, you should be fine. bear in mind, most people dont like the idea that you upgraded the memory yourself.
OSX ML - Good selling point for average consumer, but bear in mind that your purchase of ML was valid for your own apple ID, not theirs. they may run into problems in the future regarding the OS, but it should be fine.
Before you sell, its a good idea to buy your next mac (if you do get another) and use the Data Transfer utility to transfer your data. it will be like you never switched macs at all.
then, you should do a clean install of ML, this will often be another good selling point for consumers (rather you doing it than them) and also from a privacy point of view for you (really, do you want some random sniffing through all your docs and that conveniently named "Systemfolders2" that you bury deep in your hard drive

)
The rest of the price will be determined on cosmetics: does it have scratches? are there stains? does your Superdrive work properly? does your keyboard light up properly? how healthy is the battery (How long does it last now compared to when you bought it?)? Are your keys greasy? (trust me, MAJOR turn off) Do your Fans spin wildly? does it get very hot (hotter than most macbooks)? do you have dead pixels on your screen? do your peripherals work correctly? do your speakers work? did you ever spill coffee on it? On average how much did you use it in its lifetime and daily? was it ever sent in for repairs?
You will need to consider these things before selling. overall, though, I reckon you could sell yours for about the same as a higher end macbook air/lower end macbook pro. on paper, it should be worth a lot more, but bear in mind its a 2009 model, and thats 3/4 years of use already.
Good luck with your endeavours and let us know how it goes. I might be looking to sell mine soon anyway.