I am sure I don't qualify to be an AFOL (Adult Fan of Lego) or ALE (Adult Lego Enthusiast) - the Lego speak for such like minded people - because I have specifically bought only 6 Lego sets for the sake of investment, and not for assembly or play:
Star Wars
1. UCS Millennium Falcon (10179) - definitely NOT a fan of Star Wars, but this particular set's investment value as in ROI has been astronomical!
Special Architecture/Sculpture Series
1. Statute of Liberty (3450)
2. Taj Mahal (10189)
3. Eiffel Tower (10181)
4. Tower Bridge (10214)
5. Sydney Harbor Bridge (10234)
I have been told by AFOLs/ALEs that they have not come across anyone else with the complete collection, as above, of the Special Architecture/Sculpture Series - by virtue of the fact that I have an ultra rare BNIB Statute of Liberty released in 2000. Even more rare is the monochromed box - not the colored version that was released later.
All the above sets are BNIB, never played or assembled - I won't know how lol.
The current street value of the above architecture/sculpture series is the full list price of Omega's DSOTM lol. The book value's about the price of a second hand cheapest Seiko Tuna.
If anyone care to trawl around Lego forums, you'll discover retired and rare Lego sets make great investments!
To be a sound investment, a Lego set has to satisfy the following conditions:
1. BNIB - internals never opened, sighted lol
2. Box condition (preferably with outer cardbox) should be with minimal or zero creases, dings, dents, and minimal discoloration
3. Completeness - manuals, certificates, factory material etc must be intact
And Yes:
1. You can't buy them in Msia - many Lego shops will just give you a blank stare
2. You can't FeDex them - fragile, huge box will dent or break (few thousand pieces in box) thus losing ROI value
And No:
1. Your kids must NOT know they exist - all hell will break lose if they do lol