I hate to point something like this out, but the Monday after Avengers
hits the theaters is the wrong time to be telling people to get into that market. You're not going to find any supply worth having, at a price that can be profitably resold. The Avengers movie was possibly the most heavily hyped film in history -- consider that the film premiered in the US on the 11th of May 2012, but the very first teaser promos were inserted into the first Iron Man film in 2008.
Sorry to have such a negative attitude, but telling people to jump on the Avengers bandwagon now is like telling people to buy a stock two days after its IPO because it's had such a good recent run up. Yes, the stuff is selling, but most of the people selling any serious amounts of Avengers-related products were stocked up well before now.
If you want to see which movie markets to get into, you need to be looking at countdown sites, fan sites, movie buzz sites etc. to see which films are gaining gravity ahead
of their releases, and look for suppliers who will take pre-orders on merchandise. Movie toys in particular are manufactured in "waves" and by the time they get to second tier suppliers, what's left is the stuff that Wal-Mart, Target et al. couldn't sell on their closeout shelves.