We only ever had one decent lot of 'used' birds which came from a deep litter system and not battery.
Any other e-commercials we have had either came with horrendous behaviour problems; feather pulling, vent pecking, egg eating etc, or had a high mortality rate. I won't have them now, it's not worth the hassle. When you have to neck egg eaters because they even get a liking for a mustard egg with chilli powder added it's time to admit defeat. One of them stood looking as if it was going to die for three days after eating a whole eggs worth of mustard and chill and once recovered, went straight back to egg eating.
I came to the conclusion that the deep litter birds were probably kept better then the caged birds so when I was offered ex-free range commercials by one of these rehoming charities a few years back I thought, why not, and gave a generous donation. Maybe they weren't free range as claimed, but they were weak and all died fairly quickly.
It does seem a bit hit and miss and makes me wary of what else I might be bringing onto the plot with these birds so I won't touch them now.
There's definitely a case to be made for seeing where they've come from before you buy at any rate.