Except - going back to the original point - no-one ever actually does this with their carrier, because it doesn't make any money.
It's really hard to get it to make money, too, without it being a super-powered way to earn money while sat on the pad, the way that the rest of the outfitting and shipyard is structured.
Say you can sell a module to a player for some sort of markup - so e.g. you buy the module at 90% of NPC cost, sell it at 95% of cost, pocket the difference. That'd be great - an actual reason to buy the packs other than for the occasional deep space carrier.
Modules can be sold for the same price they're bought at, though, so you get the following loop:
1) Sell really expensive modules that you buy at 90% and sell at 100%. 8A fuel scoops are best since they cost about 250M each, so you make 25M on each sale, even with the one-per-ship limitation.
2) The purchaser immediately sells the module back to the carrier at full price (which you don't pay for, because otherwise docking with a carrier and selling it a bunch of 8A fuel scoops could kill its budget, the money for this transaction just appears from nowhere; hopping over to a nearby station to do the sale is also possible if that somehow gets blocked) and is now back to their original balance.
You've now made 25M out of nowhere, repeat until you run out of the expensive scoops, then sell and rebuy the pack to restock them and carry on.
With ships themselves which only get sold back for 90% of original cost there's a little more room but a 10% ship discount is still pretty marginal - any LYR system beats that.
Cut the sell-back price for modules and ships to 50% (or even 75%) and there's then a bit of room for FC owners to make a profit by selling them cheap ... but it's an annoyance for everyone else because every misclick in outfitting has to be paid for.