My shop indicates that I have 3 open slots available: "Inventory (9/12)"

However, when I attempt to deposit additional items for sale, I receive the following message in trade chat: "Dispenser 63: This is a EXCHANGE session, I will give you 0 items, you cannot deposit any more items. Click READY and continue. 11:58:27"

I thought this might be because the inventory of the dispenser 63 bot was full, but it only appears to be at 39/50 items capacity.

Any ideas what could be wrong?