I recently found a bug in skyblock that I suggest we fix, this is in regards to tradeshops. If your tradeshop sign is on a chest, then it isn't a problem. However, if the sign is on a block, anyone you trust can break the block, which in turn destroys the tradeshop sign. Once this happens, your tradeshop isn't private anymore. I suggest we fix this bug.
At this time it seems like it will be fixed when the dev fixes it. And we don't know when that would be.