I understand your points and agree mostly. The guy seemed to really have wanted more than what I paid him but I told him I would pay £330 all in. He wasn't having it so I gave in after a few days of chatting with the man and went for £350 for the gun, the other money was for postage and for paypal fees. (the fees are always worth paying I suppose, for safety) and postage because I don't drive and don't know anyone willing to drive me 2 hours to get me a toy gun lol.
I plan on reducing my collection soon aswell so the sales of those 3 or 4 AEGs would basically cover the cost of this whole gun anyway, which I'm okay with.
edit: Regarding the "knock a tenner off" bit, yeah I don't understand people who do that, that's dumb, but this one wasn't really the same case, otherwise I probably would have just put it together myself. I'm not so bothered with this purchase, I'm getting something I want, the seller gets his money, everyone involved directly, seems happy enough to go on with their lives.