I usually didn't sell my stuff because they might be used for requests, and if you are going to sell stuff it might be better to temper them with some junky shard to raise to value as long as it doesn't give it a bad quality.
You could always dualize as much as you want for better stats, but I never bothered with that much.
And to raise item slots in the mixer you just need to keep making items, I believe how it works is that the higher the eleth cost to make the item the more "exp" the mixer gets though you still need to balance with having a decent chance of maker the item.
To raise eleth capacity you just need to keep cooking, that's the only way to raise the maximum eleth, however it's not always the case that a higher eleth cost means higher max eleth raise, but it's not a bad indicator for it. Just make sure it's not something like "Revive KO" or "Party HP < 20%" where it won't happen very often in regular battles.
--
O........O
...^^^^^...