Interesting idea DM, very generous too.
In my experience once someone has decided, they've decided and unless a wheel falls off on the forecourt they're taking the bike home. Perhaps we need to get stung to start thinking properly?
Experience is the best teacher, and some people can't be helped, but at the same time, for those who pause for a second before buying a shed or who want our help, making it available in whatever form is sumat I feel most people on here would support. If it saves a few people from making the same mistakes most of us have, then that is a good thing in my book. It is so easy to buy a lemon. And pushing a bike is never as much fun as riding
Greg - aye, it's the attention to detail you learn and that often will save you. Good spot on the spare set of wheels and fuel can. I wouldn't necessarily be put off by the fact it was a trackbike, but more by the fact that the chap was trying to hide it. Obviously a trackbike is worth less, sometimes significantly less, and to stick all the road gubbins back on it and try to flog it off as a road bike is cheeky.
Miki - Good points. I particularly agree that patience is key. Finding the right bike, like anything in life, can sometimes involve hassle, expense and time. However you save in the long run. My first (proper) bike was bought the day after I took my test as I had to have a bike. It was down the road (and had been down the road many times) so location won out over common sense, knowledge and experience (neither of which I had back then).
*MOT - an MOT is not a guarantee that the bike is mechanically sound. However it is required. You want a long MOT on any bike. MOTs can often be costly, so getting at least 8 months to a year of biking in, theoretically, before you have to take it for an MOT is advisable. I would not buy a bike with less than 8 months MOT and ideally I always look for a full year's ticket.
Old MOTs can also give you an indication into the sort of life the bike's had. They record mileage, which helps to verify what you are reading on the clocks is accurate (although, like anything, there are ways to fiddle it), and they allow you to infer how the bike was used - few thousand miles between MOTs - chances are the bike has seen little rain and was ridden by a fair weather rider or possibly been on a track. 10k or more per year and the bike was used year round etc.
Owners - number of pervious owners. Bikes are like women. You wouldn't want a women that had been ridden by everyone and same goes for a bike

(on the flipside, you don 't want a virgin either - running in is no fun

) Seriously though, the more hands a bike has passed through the more likely it is to have been down the road, abused and almonded about with. Depending on the age of the bike, I like to have as few pervious owners as possible. There are some exceptions to this rule - 125s are used for a purpose and then sold on usually so they can often have been in the possession of multiple people. Often bikes are viewed toys by a lot of people. That means they often get bought and sold and can rack up owners quickly. One or two owners from new is preferable and in my exeperience means a bike is more likely to have been looked after.
HPI - 83600 text any reg to that number, costs about £3, and it will tell you if the bike has been subject to a claim, number of owners, lost, stolen, changed colour and give you the last digits of engine and frame numbers - check these against those on the bike.
Receipts/paperwork with a bike - these act as sort of history. Often people will stick everything in there. For instance, I have viewed a few bikes and discovered that they have had new engines fitted when the paperwork for the engine or invoice for fitting has been in there, or had bikes where one side had had rearsets which tells me the bike has been down the road at some point. A bundle of receipts is sometimes worth going through. You can either use what you find to negotiate money off or walk away if there is something in there you did not like or that you believe the seller deliberately failed to declair.
Buy from people - I went to buy a car recently and the chap lied to me. His story kept changing. First it was his car and his mrs was insisting he sold it as he had too many, then it was his mate's car and various other fibs/inconsistencies. I would not have bought the car anyway, as it had an oil leak, but even if the car had been mint, I still would have walked. There is no reason to lie when selling a vehicle unless you have something to hide. Ask a lot of questions and get a feel for the owner.
Service history - it is not a cast iron guarantee that the bike is in tip top condition, but as a rule of thumb, a lemon usually comes without any or partial at best. Bikes require looking after, some people can do this at home themselves, and some don't bother at all. You can buy good bikes that have been well looked after that have little or no history, but having service history can help you to decide how well a bike has been looked after. It shows someone taken time and pride in their machine and even loved it - all good when buying a bike - as it shows they have kept up with the manufacturer's requirements and spent time and money on it. It goes without saying, I'd much rather buy a bike that was someone's pride and joy and one that they had lavished attention and money on, that get sumat someone has thrashed about or neglected. Having service history is not imperative if the bike has obviously been well maintained but it is still good to have and something to look out for when you first start buying bikes. Check through and look at when was doen and when.
V5 - no matter how good a deal, never buy a bike without one of these.
The engine and frame numbers are also printed on it. Check them off against the engine and frame numbers on the bike. If the frame number is dodgy or does not correspond then walk away. If the engine number fails to match, then it means the bike has had a new engine. I would still walk unless there was a receipt for the engine as it could be stolen, but often people will swap engines and not bother to inform the DVLA.