How can I find the value of a LEGO set over several years?
Not sure about several years, but Brickpicker covers one full year. Here are the stats for the Green Grocer, for example:


  


Or the ones for the UCS Millenium Falcon:


  


And so on, you get the idea. They have more and other stats, too&nbsp;&mdash; e.g. the following quick overview for every set:


  


They pull their data from eBay, through some third-party tool they licensed.

Now, I think they only let you look up like three sets at a time&nbsp;&mdash; then you have to register. But the registration is completely free. They cover their costs via affiliate marketing, i.e. links to Amazon and the like.

They can also sync your account with (i.e. import your sets from) Brickset and Rebrickable, but I never gave that a try. I'm not much of an investor anyway, just your typical AFOL. In fact, according to them, the prices for some of the discontinued sets I still have unopened have actually plummeted.

I suppose if I were in for the long haul, I would just take a screenshot of their stats every twelve months and then string them together in GIMP.