For on going success in selling fountain pens, you do not need to become a nib meister, but you do need to know the rudiments of tuning.
I don't fill every pen I make, but I do give it a good dip in an ink bottle. While I am looking for scratchiness and smooth writing, I am also making sure the ink flow doesn't stop after a few words.This can be caused by a few things, and should be addressed before it goes to the client. I have only had two "kit nibs" that I had to replace because I couldn't fix this issue. Once I am satisfied, it gets a good flushing and drying in my salad spinner.
As was mentioned above, experienced users will do minor tuning, but if the problem is deeper they will call for support. This is not acceptable out of the box. A newbie, with this type of problem, will become frustrated very fast. They will most likely not be able to get it going, and will likely return it for repair. This wil be expected at your expense.
Avid fountain pen users tend to have multiple pens, I know some that have hundreds. The out of box experience, is a huge deciding factor in wether they will be back or not. It is well worth the time to do a little research, watch some videos and practice. I have a long way to go to become an expert tuner. While it sometimes takes me a while, every pen I send out the door writes well. If a problem does slip through, I make it right.
While repairs and re-tuning down the road can be an added service, no customer will accept having to pay extra for their pen to write well out of the box.
Just my two cents. I will see if I can dig out a few links when I get down to my office.