I have been playing Geneforge 1 for a while now, and still can't figure this out :\

When you're in a shop, all your items on the right side of the screen should have a little button that looks like two coins or two gears. Press that to sell the item.

Also - the number next to that icon is the price to sell every unit of that item. Took me a while to realize that.

Beware that each vendor has a finite amount of money available, so after a while you might need to find a new one to dump your stuff. The only way for them to get new money is when you buy something from them, it doesn't regenerate on it's own. Also note that you can pretty much keep stuff laying around on the ground for later use. No one's gonna take it and it won't decay. The first village has a house on the north border which may become uninhabitated due to your quest choices. That's the place i store my spare things in.