If you willingly allow a second person into your market booth and they steal your items, do not expect to get them returned. It is your responsibility to protect the items in your stall. Be sure to have your group turned OFF at all times, to stop people grouping and approaching you within your stall.
"Market display" ends the possibility of theft in the market when used properly. Justice will not handle market theft cases unless there is a screenshot admission of the theft.
Contact a fellow player if you have questions about how to use "Market Display". This option can be found on ALL NPC's within the marketplaces.