Sure, there is always the option to go sell stuff and come back later.  However, that's a pretty poor solution.  Traveling out of my way to get rid of junk cargo?  That's exactly the sort of thing that I'm trying to avoid with dumping goods right on out.  For example, say I'm mining out a system, and there's just this one planet with a bunch of exotic elements.  By the time I get all the way to the nearest discovered trader to get rid of the useless stuff, I'm not going to go all the way back just for that one planet.  There's so much exploring to do in this game that something has to be truly exceptional to backtrack (read: not a planet full of resources).