Some interesting info and no doubt some useful advice for Gazza. I've only been doing this for a year now and I'm pretty small fry but I am fairly business savvy (in addition to loving lego) and in my short experience Ive learned some lessons which I'd like to share.
I did get some Dr Who sets from Myer during the sale and I was shocked at the speed at which they were gobbled up. Furthermore, on Facebook I see more and more (ad nauseum) lego fans posting "to build or store away?" (insert Iron Man eye roll here). I feel like to my mind sometimes I'm going from bull to bear in the course of one day. It's strange times indeed. A race to the bottom perhaps?
Ideas and modulars are way too hoarded, and, while I believe that money will be made on them, I don't believe that 100% ROI will be achievable. Alucard has addressed this topic a few months ago and, while initially skeptical with beer-goggles on, now I believe him to be right. The days of doubling in value will not apply to these IMO. 12 month cycle? You can maybe tell that to my one Birds set, but not my 11 Ectos (which used to be a higher number before I liquidated a few at Xmas). Dr Who is almost literally flying off the shelves, so why would TLG pull the plug on it? Perhaps they got a 4 year licence for it? I've never ever known of a company that makes such strange business decisions as TLG so WTF knows. Maybe the Delorian has tripled in value, but what about the Exo suit or Research institute? They've made money, but they are really small sets and you would have to sell a few to make any decent coin.
At the beginning of last year, my targets were Palace Cinema, Pet Shop and Ecto. All are still collecting dust. There's a lesson to be learned there.
Dark Years has a point about international resellers. It is a real phenomenon that's a perverse form of globalisation as well as being a massive waste of fossil fuels. However, 11th hour purchases at retail price for just-retired sets has worked well for me. You need to do some legwork though. Even those sets haven't doubled in price yet but we'll see how they go. New listings seem to appear frequently even for highly-prized sets that were retired after less than a year. I can see many sets plateauing indefinitely.
My biggest wins have been smaller sets which have flown under the radar and most of which do not have their own thread on this forum. I won't say what they are because at this stage in the investing game I reckon it's better to use the info you have and keep your cards close to your chest. I can tell you, however, that over the last year my return on investment (after expenses incl fuel) has been anywhere from 15-100% but no more than 2x RRP, with an average of 40% and a lot of time put in hunting, stalking forums/blogs, packaging, listing items and kindly telling GT lowballers that my price was specifically stated as non-negotiable etc...