Let's take a look at Hasbro's The Black Series 6" figures on the secondary market. Most Black Series 6" figures can be had for regular retail price even years after their release, but there are some figures which have increased in value gradually.
Currently the top spot goes to the Boba Fett/Han Solo in Carbonite 2-pack which was a San Diego Comic Con exclusive in 2013. The demand for some of those figures is pretty amazing, even bootleg versions seem to be making quite a bit of money on the secondary market (Darth Maul!).
Many of the figures listed below have doubled in value, if not more, check it out:
1) Boba Fett / Han Solo in Carbonite
2) Luke Skywalker (X-Wing Pilot), 1st release and 40th Anniversary Collection
4) Darth Maul
5) Imperial Shadow Squadron 2-pack
6) Bossk
7) R2-D2 (first release and 40th Anniversary Collection)
8) Yoda
9) IG-88
10) Luke Skywalker (Jedi Knight)