Daniel,
You were exceptionally fortunate with your purchase.
I would suggest that your good fortune is still more the exception than the rule, even in this climate.
Airheads can be had for the price that you paid for yours, but ebay is probably not the place to to get them, unless the adverts have been poorly placed or described (like my £700 R100 - BMW Black/Blue). However, even my R100 needed work.
Ebay, often brings out the bidding fever, especially when similar items have been thin on the ground. Like airheads in the last couple of weeks.
A friend of mine has just bought an ex-police R75/6 of indeterminate mileage for £800. It runs and has an MOT, and a single ATE swinging caliper up front

. On the face of it, not the greatest of purchases - slightly over priced.
But he wanted a traditional bike with wire wheels to throw the spanners at - a 'rolling restoration', as such. It fitted the bill and was local (1/2 a mile away). He was able to pick the seller's brains and view and ride it. It came with some spares and quite a bit of history. And it all works.
It was too easy. A couple of wasted treks up the motorway and he would have been out of pocket on any deal.
Unlike a similarly powered/braked Norton/BSA/Triumph.....Which would have cost him 3 times the price for a scruffy runner or basket case.
Looking at it like that and he has exceptional value for money, even though it will probably never be worth anything more than he paid for it, as it will always be an ex-police bike with indeterminate mileage.
So while I agree with you, that doctoring the mileage on any bike is never right and the culprit should be brought to book. I would also suggest, that even taking that into account, the bike was not outrageously priced, if the general condition was as good as it looked.
Steve H