It's a pretty simple and probably obvious idea: give more items sell effects.

Imagine you are going through the shops, and one of your options is just [Massive pile of gold]. Maybe it has a combat effect to gain some money, but the key part is that you can sell it for 5 coins. Buying it and selling it right away is a 3 gold profit over skipping that shop. However in order to do that, you need to have three spaces open in your inventory.

You could have items which give your other items sell effects. I would make it be a one time effect rather than a passive, to encourage a sell off of several items rather than just cycling stuff through an open slot.

There should also be more sell effects. I noticed several in game, stuff like the ivory tusk which gave a weapon attack or the ship, but it'd be interesting to have stuff like a healing item that when sold restores you to full health (useful if you take a beating from a monster), or something that messes around with your health or hero power. We haven't seen all the cards so there are probably some of these on the drawing board.

There could be a rare merchant, who would let you sell items for a substantial bonus. Possibly for something other than gold, such as upgrades or even maybe player levels?

I think it would also be good to have items which you are encouraged to hang on to through combat/upgrade, such that it isn't just a buy and immediately sell type item. I'd rather have these be useful items during that time, to distinguish them from the existing items like the treasure cove which are just dead weight in fights.

All in all I think the current version of the game is looking fantastic, and I think that leaning more into the selling side of the game will help with the feeling of being a merchant in the Bazaar.