What's wrong with it is that items cannot retain value if they have drop rates that most people would consider "acceptable" for ssf play. With a free trade system, and a large enough playerbase, items would quickly drop to values low enough to where it's multitudes more efficient to just buy every item you need instead of farming for it.

This causes a problem where, if the market is taken into consideration at all during balancing, drop rates will have to be affected. How do you have drop rates balanced so that one person can farm for a chase item within what's considered reasonable, while also having that item retain value in a market? You can't. So the ssf player (and honestly, everyone in general) is affected in the end.