I'm gunna weigh in here and totally agree with throwing guide books out
Examples I'll give are the discovery I bought for the mrs for one. Booked at £30k forecourted at the main dealer for £28k as he was over stocked. Cash purchase and he took a further 6k off the car, and threw in loads of extras to complete a sale. That's £8k difference.
Sold my .:R32 at the start of the summer, Booked at £15k sold it privately for £24.5k, demand outstripped the supply as it was one of a kind, bought by someone who didn't even come and look at the car and then had it delivered. But for such a car there were people happy to pay the price, all offered the asking price.
My Defender that I sold the week before Xmas booked at £15.5k forecourted at £22k I sold it for £18.6k to a chap that did a 1200 mile round trip to come and buy it, it was only slightly modded but enough to set it apart from the norm - blowing the myth that mods don't add value out the water, a favourite dealer line sometimes. Mods if done right can make a car attractive and easier to sell, eg two Gti's next to each other and identical but one has bbs rims.... Which one you gunna buy
Sold the mother in laws Audi Q5 back to the dealership in December too, bought for £40k a year earlier and they bought it back for cash for £39k, a case of demand outstripping supply.
As for forum cars? I've just bought a forum car, collect it this week. The member had a high post count so I was able to read back through posts they had made on the forum and find out massive amounts of information on the car that sometimes stay hidden. For example if it's been damaged, broken down, any mods, issues with performance etc. when people have problems they post, when they mod they post and this all adds up to a history you can read before buying the car. It's enabled me to virtually look round the car with photos and so buy it remotely without travelling to view just to collect
In a nutshell, if you want a certain car you have to pay for it, if the seller knows your want it they won't budge on price or if they have other genuine buyers in the wings. But if the car they have is merely stock to them the margins will always work in your favour, if they say they have others interested then they would have sold it already and only a foolish salesman would let a good sale fail purely for a margin of movement on the price