Hi Seargent,
If it's not to do with the marketplace, there isn't a lot we can do without an explanation of what happened.
Could you please see if you can provide some form of video evidence of you losing this item, or if you can, recall anything which may have happened to cause you to lose your items?
It isn't possible for your items to just randomly disappear from your inventory without any action from you, the user. You may have accidentally traded or sold the item(s) without realising / remembering it.
Thanks!