Besides the non level up feature this game has, and I hope to be wrong here, you can't sell your extra weapons, I mean... really?
Some tips by my side:
1. You can upgrade the armors buy paying, it's not a big difference but it can help, I recommend doing it after exploration, so you can unequip, upgrade, then equip again and then going to sleep, because the boy's armor give hp, so unequiping will cut your max hp.
2. You can save before going down to another floor, if the floor doesn't have any reward, may just load and go down again. Very stressful to do all the time but it's an option if you have no food nor liquid and want some good items before going out. May help a lot in early game too if you find a nice blade or armor in first floors.
3. Ecchi scenes and battle take some energy (food and water), so keep your ecchi time at minimum, unless you don't care.
4. It's possible to defeat the first boss at the first day, though I would recommend to find a wand for the boy, he will be better in magic than with blade and has more MP.
5. There are stats-up items but are very rare and you only choose one to get the bonus, think in your strat, I personally recommend giving magic to noble and strength to maid, don't know which stat gives hp, the noble may need some of that too.
My comment: This game really needed a quality check, the "scenes" are too much repetitive and story is lacking. I was really hoping this game could be awesome because I am hoping for a "chad male protagonist" or " good male (no shota, no passive) protagonist" game.