If the game ever changed to have a player driven economy and you couldn't just buy an infinite amount of resources from Sol then I would definitely agree that this would be a great feature to put in. As is however I feel like it's pretty... unnecessary. There are simply a great deal of other things that need to be worked on ahead of this. Still it isn't a bad idea, just the fact that you can buy whatever resources you need for building at any time you want back at Sol makes it kind of not mattter whether you sell something or not. Unless you were selling massive amounts of resources or very rare materials then the amount of money you lose out on is pretty insignificant.
Hmm. I'll try to detail what I'm going through...
I do a lot of mining because I like too. I get a lot of minerals/gases this way. I haul them to SOL and sell them. The past couple runs I just sold what I had without looking. However, everytime I arrive at SOL I upgrade for several days using the solars I gained by selling. After looking at my manufacturing screen several dozen times, I started to see I sometimes have sold minerals/gases which I could have used in manufacturing and/or kept a small supply of them for repairing armour. So here I am at SOL wanting to sell a big haul, yet I'm looking at my manufacturing screen and wishing I could mark them to make it easier to sell. The only option I have now is to memorize the minerals/gases which makes it harder.
Example... Cobalt sells for $275 and is purchased for $330 at SOL. Lets say I have 100 in my cargo on arrival and (stupidly) sell it for $27500. Several hours later I buy 100 Cobalt to upgrade something for $33000. I lost $5500 in this transaction.
Inevitably I have to plan ahead before I sell. This means looking at my manufacturing screen for some time to know what the minerals/gases are, since I don't have a photographic memory. It also depends on what I'm upgrading and how much, since $5500 isn't a great loss. I also sometimes have other ships in the vicinity which I can upgrade as well.
If the sell/buy price were equal then none of this would matter much, since taking the time to mark anything and keep track of how much I need of each mineral/gas would eat up more time than just selling and buying impulsively.
In the grand scheme of things this is a small thing. And you're right, there're lots of things which could be done. And yet a lot of the problems this game has are related to this problem. It's an interface and/or management-overload issue. One example is when I want to upgrade something I have to know what and how much. There's no way to setup a purchase via the manufacturing screen to ease the burden. I have to memorize or write down what I need and how much. (*)
I've taken it all in stride. Yet I've made a dozen documents at least to aid me. I literally have a document which lists all the modules, their costs and how much I need at particular MK levels. I do this to make it easier for me when I'm purchasing.
* - I imagine it'd work something like this... You go to Manufacturing screen and click on one of the modules to Manufacture or click on Mark Upgrade. Then when you enter how many MK levels you want, the computer would calculate for you how much you're missing and would then offer you the ability to automatically purchase it either via the Salvage Ship or a SOL station. Alternatively it sets up preorders and you have to go to the Salvage Ship or the SOL station to execute the order.