There are a lot of questions around the who what where and why of retired sets and a lot of it is a mystery to collectors and enthusiasts. I tried to answer the common questions that I get asked on a regular basis regarding lego collections that are retiring and for the ones that I couldn't answer I reached out to Lego for an official response. This should help you in your hunting.
LEGO is quite secretive about how and when they retire sets. They will flag some on their website, but frequently sets leave production without notice. Their official comment is that most sets will remain in production for one to three years. More popular sets will stick around longer. None of this really helps to answer the question, but that’s because LEGO doesn’t want it to be widespread when they plan on retiring each of their sets.
Keep checking back with BrickScouts.org for a continually updated list as we try and keep track of them on their own page sorted by year. Here are the Lego Sets being retired in 2017 . Updated April 30, 2017
This is entirely dependent on the set itself. A retired set in huge demand could be worth many, many times more than its original retail value. Some of the most popular sets can go for thousands (yes, plural) on resale markets.
LEGO can retire a set at any point in the calendar year, there is not a specific timeline during the year for this to occur.
Ideally, we try and find them in a store or online before they are out of stock and that was the entire point of creating our retired set tracking page. This will ensure we are paying retail price (or even less) for the set. If a set has been retired for some time then your only choice is likely the secondary markets online. We highly recommend checking these eBay listings, and Amazon's New & Used section.
LEGO retires sets to make room for new ones. They are constant churning out great new sets and don’t have an unlimited manufacturing capability, so something has to go in order to make room for the new sets to be produce. We asked lego this question and they told us "It is purely about production and keeping the lines fresh. We have to produce new sets so people will continue to be interested and we can't do that with a finite production line."
With any collectible item, the harder they are to find, the more people are willing to pay to own one. Once LEGO retires a set, they have stopped manufacturing that set so no new copies will enter the market. This puts a hard limit on the number of sets out there which drives the price up.
If LEGO released ten copies of a specific airport set priced at $80, the price would be $80 until they all sold out. If LEGO retired that set after those ten copies, then only those ten copies would be available to the entire world. Five of those copies were purchased by unknowing parents for their kids, they get mixed up in a ten gallon tub of LEGOs never to be heard from again. So there are five of these airport sets left in the entire world. How much would a LEGO collector be willing to pay for one of five sets in the world? The sky's the limit. This is an extreme example, of course, but the idea is the same; once LEGO retires a set, quantities become limited and the value to collectors increases.