Over the past year different things have happened in occurrence with this type of thing. The first instance I've seen and personally experienced was that when an auction was won with a full inventory the items would "drop" onto the ground. Only they would be ghost blocks and could not be picked up or seen by other players. There was no warning message. After some work was done after this was becoming an issue- the server started relaying the message that your inventory was full and your items have been dropped on the ground. However it would appear that only one stack of the item would be there no matter how much more there was or none at all would. Did the test today with a full inventory. Got the message some items had dropped but none were there.