hey! thanks for the question. you should be able to sell stuff now at the merchants. Maybe there is a bug but the items should appear in the "sell" list and you can scroll it to see everything.
We also have the "base" funds which you can now add when doing an objective loadout (if you have any funds at least). We are still working on a few more ways for you to spend money in the base. Buying subs, buy/sell equipment (merchant) and hiring more permanent team members are the working ideas for right now.