Dark strip down center of page when coping from tray

There is a small strip of glass on the left side directly under the edge of the scanner, About a half an inch wide. It has something on it.
May be very hard to see, use a strong glass cleaner or some alcohol and a regular piece of paper to clean it with.

I own 2 MFC-7340's. As described in a previous answer, a vertical black line down the whole printed page is usually the result of an ink spot on the 1/2" strip of glass. I like writing with gel pens but they leave little gobs of ink and the script takes a while to dry. Ordinary ballpoints can do this, too. These gobs smear onto the glass scanning strip and that causes the black line on your print from the gob downward - and also on every succeeding document! Before you feed your documents, use the heel of your hand to blot any fresh script and then lightly sweep your fingers over the page to disperse the excess ink. The rubbing will cause some minor smearing, so if the image must remain sharp, just let it dry completely or (at the risk of putting dots of ink on the flatbed, which could potentially transfer to the white pad above) scan it on the flatbed. If you get a stripe, a dab of spit and a few wipes with a finger across the glass usually get rid of it. Take note if you do a lot of scanning, you should view a sample scan onscreen occasionally to make sure you haven't picked up a stripe.