I'm not really a fan of beating about the bush when it comes to negotiating price.
If it's up too high I'll tell them and show them examples of very similar cars going for less money.
If they refuse to budge on what might be a reasonable/slightly inflated asking price then any imperfections will need to be rectified at the sellers expense (wheel refurbs, bodywork correction etc).
Any defects/imperfections are taken in to account, along with MOT/Tax & tyres, before putting forward an offer having asked them, what they are realistically looking to fetch.
Eg. if it's on for £10k and they said 'I can take a hit of a couple of hundred due to XYZ but not much more' offering £9k is just wasting both of yours time. Don't get me wrong everybody loves to get a good deal but if the seller isn't in a rush and really thinks the car is worth what it's on for then you're at a bit of a stalemate.
That said, there's no reason not to negotiate, it makes you as a buyer feel like you're getting a good deal. Recently I negotiated a new MOT, full tank of diesel, rear bumper respray/correction, 2 alloy wheel refurbs and a 2 year warranty along with the expected valet/detail. If they want the cash sooner rather than later then you're in a position to ask for a slightly further reduction, but not much.