My own rules for investing in Lego.

The biggest danger I can see in all of this is fakes. You have a product with a high value which lots of people want so obviously people will want to reproduce it. 

Although I could potentially make a bigger profit from buying from online markets and auction sites, you leave yourself open to the huge risk of getting a fake or getting something not as described.

All the sets I buy will come directly from Lego, so I know they are in a factory brand new condition and original. This premium is worth it as I am sure it will be repaid in the future with the knowledge that you have the real deal and you can ask a higher price for what you are selling on, having that providence.

So the first rule of investing is, we only buy from Lego. The second rule of investing is ensure you follow the first rule.