I'm a low enough rank that I have credits sitting around so buying, selling and refitting modules with completely disregard for credit costs is common. I also have a handful of uninstalled mission modules and stacks of artifacts cluttering up my tiny cargo hold.

A lot of the time, I want to build 3 or 4 of ABC module, so I need 3 or 4 sets of XYZ minerals. BUT I don't have enough cargo space to hold it all at once!

So I buy 1 set of minerals, build the module, go back to the trade screen, put in the number I want to buy, buy the minerals, go back to the ship screen, click on Build new module, scroll to the one I want, and build it. Rinse repeat.

It would be nice if it would automatically buy the minerals necessary (either the whole lot, or the remaining necessary if you have a portion already in cargo) and just get on with the job. A confirmation beforehand might be necessary to keep the click-happy people safe from themselves...

Or maybe I'm the only one experiencing this because of the way I (inefficiently) swap my modules around depending on what I want to do?

This should only really be a problem if your still using dynite, in which case you'll grow out of it.

Otherwise, scrap some artifacts.

I wouldn't mind seeing this happen. It's been suggested several times before. Although I would prefer it being put in place for planet structures first. How many times a juicy planet gets shared, several people fail on it then someone (who never seems to have minerals on hand) rolls a good invasion... Then it's a scramble to get new defenses put in place before the other legion has a chance to come after it.

Autobuy with confirmation, just display the final price of module or planet building + minerals.

" Would you like to buy XYZ for a total cost of ABC ?"

I like this, but add one thing
the system need to perform a cargo check to see if you have enough room in it for that little bit of mineral for that ONE module.
I don't see this as a problem by the words of the OP, as he just hates buying for one (set of mineral for a module) at a time.
However, it would be a "cheat" if there was no cargo room left at all.

been suggested several times the first time by of +1

Oh yeah, it would definitely check for cargo space for the minerals first. I meant it would work the same way as if doing it manually, just... with about 6 less clicks. Computers are meant to make our lives easier, right?