despite being my favorite band, a good example of selling out is Linkin Park for me with their last two records (R.I.P Chester)
The Hunting Party was a thrashier and much harder album then most of their past releases had been for about 6 years (In a sense you can call those three albums post Meteora as also 'selling out') and then they went and released a straight up top 40 geared album.
I'm bias and loved all of their work but even I can admit that that's selling out hard for the extra cash