Off the top of my head..
AUTOart - MB 600 LWB, the last reasonable priced one I saw on eBay was about £150 (I think) At the time Preisman was looking for one, but the one I'd seen wouldn't send out of the UK, I said ask anyway, voila! He scores a 600
CMC - MB W196R Plain bodied streamliner, MB World had it for £150 - but at the time I didn't know if that was a good price or not.
Exoto - Ford GT40 and Sauber C9 sets, I've seen both for great prices, but they still weren't cheap, so I didn't get them.
Exoto - Nigel Mansell Ferrari with 'tear off' visor for £100, I don't know about F1 cars so I didn't get it.
Minichamps - Andy Warhol BMW M1 Art car, I missed a couple at £150, I wish I'd have got one! I don't even remember them coming along when CK etc had the Art cars for £70
Edit, a couple of others..
AUTOart - BMW E30 M3, either the Soper or Cecotto one, for £70 on buy it now. By the time they'd confirmed it was a 1/18 it had sold.
AUTOart - Mitsubishi Lancer Evo 9 'Ralliart' New in the box for £150, this was about five years ago now, it was local and only had a stock pic, so I asked them if I paid now could I collect ? They said no and someone else bought it.
All of these were a fair while ago, I'm not that naive now. Mind you over the years I've had more than my fair share of scores