I have a good paying job but got into this one small step at a time. I had read articles about people in the Toronto area who were doing this full time and between my love of lego, comics, star wars, and having a son who's also interested in this stuff, I figured I had a good idea of what would be worth money, which was supported by my research into what some of my personal sets were worth if still in their sealed box. The eye opener for me was a few years ago when TRU had a promotion where you spend $35 on lego and get a free robin mini cycle polybag set. I ended up with 3 of these (one for me, 2 extra) and listed the extra 3 on ebay. I sold each for $65 CDN, which to my mind meant I got all the lego I had purchased for myself for free plus a little extra. So with all the trips to various toy sellers I was already making due to my and my son's interests, I started picking up sets whenever I saw them on discount and selling them after EOL. Now I am selling between $400-$800 worth of sets per month, and I figure close to half of that is profit.
I'm comfortable with the amount of time I put into this and don't feel like I'm really expending any extra time or money other than the time and costs of shipping since I'd be going to these stores anyway even if I wasn't reselling. Many of the sets I sell on Craigslist and Kijiji end up requiring nothing more than a short walk from my office to meet another office dweller since I work right by Union Station, which is surrounded by working parents with no convenient toy stores nearby. And there are lots of Canada Posts down here too so the Amazon sales also only require a short walk. I don't ever see myself giving up my job for this, but the side money is too good to pass up given the convenience. I can see myself doing this into my retirement years (which should still be decades away) as long as lego is still popular then. And even if it isn't, I expect kids who grew up with it when it was popular will want more sets when they're grown up like me.
My short term goals are to start buying more of the bigger sets soon, like the modulars and ultimate sets. I have a lot of them for my personal collection but just bought my first one for resale last week - the Ewok village with double VIP points plus the Ninjago giveaway. I have been pleasantly surprised with how much can be made with even the small $20-$50 sets. I can get these for 40%-50% off during clearance or other promotions from Walmart or Supercentre a few times a year, which makes reselling easier. I have never had a problem selling these kinds of sets fro the MSRP after EOL as parents will happily pay that price (and no tax) for something that can't be found, and they won't feel ripped off by paying less than the after tax price. And I get close to a 100% return at that price, which is often after only a very short turn around as I find a lot of those discounts happen right before a set goes EOL.
When I was young, my friends and I dreamed of selling our comics for profit. That never panned out, but ironically I am now making money selling lego based on the comics I used to read. What a world.