They should just do the selling of Modules like they do with other Commodities in the Game, some Systems don't have ANY Grade A whatevers, other Systems do, if I want to sell a higher Graded thing that is NOT available in the current Outfitters then that should command a premium (Not saying a HIGHER price but definitely more than the 'across the board 10% loss over new'), same as there should be a greater financial Penalty for selling F Grade Stuff where A Graded is abundant.
For me though the 'across the board 10% loss' is a stupidly bad idea as all it does is quash the freedom of choice and experimentation options available, it forces you through fear of financial penalties to make a single choice and keep to that choice specially with the more expensive ships which are already overly extortionate to outfit, people just hate 'losing' money, no matter how much more money you can make easily in 30mins, it's human nature.