Totally against a black market where you can agree a personal price with another player. But, I would be happy if it was some kind of auction market with gree. Youd sell units you didnt want back to gree, and have the opportunity to make sealed bids on units you want (one per unit), maybe a list of 100 different units, (mixture of cash, valour and gold units) and you find out within 1-2 hours if your bid(s) was successful. Be a good opportunity to get rid of unwanted units. Especially if you had the option to purchase and sell units for gold/valour and cash. Think it could add a new dimension if done correctly. But with the amount of existing problems in the game, I would much sooner see these sorted before you bring in anything else that could add to them.