I've seen items flagged as "No Trade", which means you can't give it to another character on your account. And then there's "No Auction", which means that you can't sell it at the Bazaar. Oh -- and then there are items (freebie stuff mostly) that just have 0 value. But all of the above can be sold at stores in the shopping areas.
Are we talking about equipment? (Sell those in the regular shops.) Or housing items? (Sell those at the furniture shops.)