WoM had this exact same issue, I have no idea if it was fixed in the patches but basically, you can get far too much gold by selling off junk weapons and armor.

Being able to fund your entire empire and lower your tax rate by selling stuff your heroes pick up should _not_ be possible, it completely breaks the economic model and removes the (already limited) need to balance city maintenence costs with income, while giving you a boost to research and production.

The issue isn't quite as severe here as it was in WoM, but there are still some obnoxiously exploitable methods of generating cash (repeatedly killing AI armies and acquiring basic loot, then selling that all while gaining XP for your stack of doom).

Monster trophy loot is comparatively less valuable, but basic weapons and armor are selling for more than the trophies you get for killing dangerous monsters, so these numbers need to be looked at.