'It could be that the more automated & impersonal the return procedure has become....'

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I think this is a good point.

 When there's no human contact , messaging or conversation between buyers and sellers,  feeling guilty about harming a fellow human wouldn't enter into it so much...

Perhaps they feel more like they're ripping off a computer system, therefore it's a 'victimless crime' ?

Rather like shoplifting from massive supermarket chains, as opposed to nicking stuff from the corner shop where you have just said 'good morning' to the guy behind the counter!

(Obviously there's not such thing as 'victimless crimes' : we all pay for it with increased shop/insurance prices etc......but that's not how it looks at the time)