While in the shop window in Aliron (or any travel location with a store)
your inventory is the list on the left
click on the item you want to sell
a quantity bar should show up near the bottom of the screen to select how many to sell
then click confirm to sell.
As for selling slaves in the Slave Market in Aliron at the top of the list of slaves there should be a tab that says sell.
Select who you want to sell
click show info in the bottom right of the window
then click sell in the bottom right if you are sure you want to sell them.