Can of worms.
Down trading runs mostly at a loss, based on the value of materials you can deduce from forgone mission profits when taking mats as a reward. You'd better hope you don't get too many down traders
Everyone will want to trade Imperial Shielding for Pharmaceutical Isolators. No-one will want to trade PIs for IS. You'll end up with a glut of IS, and where are you going to get more stock of PIs?
The Material Traders simply don't make sense as a business in stations, but it doesn't matter there because they're really just about making getting the higher grade materials the player wants as painful as the player can stand.
They wouldn't make sense as a business on Fleet Carriers either, but there it would matter if you were hoping to make money from them.