lol K-ON!s probably going to be the best selling anime ever (not in terms of DVD/Blu-Rays, Manga, figurines, but by the amount of real life products it sells due to K-ON! XD)
Ex:
The guitar set (ex: Les Paul Cherry Burst Guitar)
Yui Hirasawa’s Beginner Guitar and Akiyama Mio’s Beginner Bass
K-ON! Let’s start a band (includes sheet music of the songs)
Electric fan in K-ON
Cellphones of the K-ON Characters
Tuners
Mugi’s keyboard
Ritsu’s drums
Azu-nyan’s cup
Ritsu's pen and bag
Mio's headphones
etc.