The 90% discount on sold items feels pretty harsh to me but at the same time I understand why Stardock made the change (to make gold more scarce).

I suggest that the selling prices start out at this discount but then trading and economics both improve the shop buying and selling prices e.g. trading makes selling at 20% value (as opposed to 10%) and you get a 20% purchase discount from the store. Economics would increase the bonus further to 30% value for selling goods and 40% discount when buying items. Similarly the construction tech could give a 20% discount on rush buying buildings.

Anyway just a couple of thoughts I wanted to throw out there