The majority of items in DAoC can be sold, traded, dropped and even salvaged or destroyed. Very rarely inventory items will not render correctly and become "invisible". When this happens moving other items around in that bag/vault will cause them to become visible once more.
If you notice an item has gone missing and the above suggestion has no effect, simply place an in game appeal and a CSR will be happy to investigate. Please understand that although we will do our very best to discover what happened, lost items cannot always be returned.