Ok, first things first, use the market as an extended inventory. If you can sell something on the MB and buy it back later when you need it, nothing was lost. Likewise, things easily bought from NPC vendors for very little money you also should not bother hanging onto.

Another thing to consider is leveling all your DoH/DoL together (and your remaining DoW/DoM if possible). The game is designed in such a way that they items a level 20 gather or level 20 battle class obtain will be useful to a level 20 crafter to make gear and items for level 20 classes. By keeping your classes at a similar range, you're reducing the range of items that you need AND the range of items you're obtaining to a mutual list. If you do leave some battle classes behind, go get the items to make the gear they use when you need it, don't hang onto copper ore, maple logs, and oranges just so you're ready if you one day level it up again.

TL;DR Keep what you either use on a regular basis or are extremely difficult to come by, sell the rest on the MB (or to NPCs if the MB price is near the NPC price.)