A lot of people on my server, and even in my guild are buying old accounts (before the battle.net merge happened) and fuse them with their own with just one click. Hackers are selling them for 50-150€ and you basically never know whats inside, it could you nothing but also a character with rare achievements, titles, mounts or pets, a lot of honorable kills or with several millions gold.
I contacted a GM about this issue, reporting everything I know... well, he said that this is "impossible" for wether reason.