What do the merchants buy from me?
Many merchants will both buy and sell. Of these, most will buy items similar to the ones they sell. So, somebody who sells shoes might buy them too. But a merchant who sells shoes maybe isn't interested in buying daggers.
Note that different merchants charge different prices for the same Items. It's likely that the merchants who charge more for an Item will pay less to buy it from you. So, explore, and ask experienced players for advice.