I didn't complain about not being able to sell heavy items. What I said was "Luckily Dammon is still there and he has enough money to buy my heavier stuff". That removed my encumbrance.
I don't want to send junk back to camp and have the ball-ache of grabbing it and doing back and forth between camp and merchant. Especially when I have not got a clue when or where I will find a merchant.
If you have Volo in your camp, you can trade with him.

Also, for your spell slots can you sleep first to restore them before meeting Halsin, or does the quest fail?