There is nothing "wrong" with it, but selling key items just doesn't sit well with me, and I view groups that sell them as detrimental to the game. Also, I'm not sure why someone who couldn't do it themselves would buy it, because if they're not doing Delve what do they really need the gear for?
Being able to perform 1 role in delve is a lot different from having a group capable of every role adequately. A large portion of the playerbase can play one job adequately. Very few have groups of 17 others that can. You are not even required to participate, it's merely an option.
People would buy the gear because doing everything significantly faster results in getting significantly more ... everything.