Yep, like I said space marines can be had for dirt cheap because there's just so many of them included in so many bundles. It's not uncommon to find someone with 8,000 points of their primary army, 4,000 points of a secondary army, maybe started another project with 2,000 points of a third army, and probably has another 2,000 points of fucking space marines sitting around on sprues if they haven't tried to get rid of them. Every edition launch box, the Hachette magazines, people getting them as prizes at events, random friend handing things off because they've got piles of them for any number of the previously mentioned reasons.
Then you've also got miniswap, trolltrader, FLG's used store, local shops, ebay, etc.
Hell, I'll even give an example that's basically guaranteed to happen this year. The main 11th edition release box(like Leviathan, Indomitus, Dark Imperium, etc) is going to end up as half space marines and half something else(all of the rumors seem to be pointing at orks). As long as you don't care about the books and other bullshit in the box, you should be able to get the space marine half for half the cost of the box saving you a bunch of money on shit you didn't want/need, if not cheaper even though it'll be new models. It's the same way with stormcast in AoS. Doesn't have anything to do with the models or army being bad, it's just that over-saturated.