can you get more space in your inventory? or what? and what happens when somebody boght an items and theres no space to put in my inventory?

If any of your trades receives more items than it pays, once the total items in your shop exceed the limit, all of your trades will not be visible for other users. You can withdraw items until your total items goes below the limit, and your trades will be visible again.

It is better to reserve one or two item slots for trades that will receive more items than it pays.