There may be more examples, but:
1) Availability of current sets. Last fall when the Minecraft sets came out, Lego wildly underestimated their desirability. With resellers scooping up as many as humanly and computer-ally possible, actual fans found themselves unable to find them in stock, the secondary market saw a huge boom for a bit, but Lego feels (and not unreasonably so), that fans and kids got a bit hosed in the bargain.
2) Availability of buyer dollars on discontinued sets. If I am a lego fan and I have $500 to buy sets with this month, I can spend that $500 on currently available sets from Lego and/or their authorized retailers (either of which help Lego's bottom line at this time), or I can spend my $500 on discontinued set from two years ago that Lego originally sold for $200. Not only is that taking money from Lego's bottom line this year, but insult to injury the $500 I spend only ever gained Lego $200 (at the time the reseller bought it from Lego or authorized reseller).
3) Brand protection. If I'm a chain-smoking crackhead with bad breath and warts who regularly treats my customers like dirt and I'm also a Lego reseller, I give Lego a bit of a black eye by association of my fugliness with their product. Of course in my case I'm a glamorous 47 year old who doesn't look a day over 23 (or, if you ask my sister, a day over 83...) with beautiful merchant manners and a winning smile, but not everyone is so awesome.