This probably has been suggested, I’m not too sure, delete if suggestion has already been made:

Perhaps we can use an auction house for items we don’t need or use. For example, we hit invaders and get these rewards, but not every item is used. Perhaps we can sell them back for a small bit of price to other players or just the store itself.

Also, as an auction house, we can sell, buy, or even bid on pre made gears, runes, gems, boosts, and etc to profit gold/goods after selling/buying goods. This can be an investment for in-game players that doesn’t spend money and it is a faster process building up hero sets. Nobody likes waiting for certain invaders just to show up and then have to wait another two weeks for a different invader to retrieve certain items. Everyone loves getting into action, but how can they possibly do when the process of building a good hero set can take up to weeks building?